Posted on 45 Comments

How to Apply a Three Coat Lime Render

A guide by Cornish Lime, using NHL or Lime Putty

There are a number of different substrates you could be working with, from a simple masonry wall to a timber lath substrate, and we have tried to keep the following guide as generic as possible.

The following guide applies equally to both NHL and Lime Putty renders, and Cornish Lime stock an extensive range of ready-mixed base coat and top coat renders to suit all applications, supplied as both NHL drymix and lime putty. We also supply premium quality lime putty plasters for fine plastering work.

Preparation is key

As with most things in life sufficient preparation is key, and when carrying out any type of rendering making sure the surface is thoroughly cleaned and free of all dust or debris is of paramount importance.

Also ensure the surface is not too smooth and, if so, first score or roughen the surface sufficiently to provide a good key for the first coat to adhere to.

Avoiding the pitfalls of lime rendering

Lime renders can be temperamental and do require due care during their application and their infancy, and can fail from excess shrinkage, drying back too quickly, or weather damage during the early stages of their set.

However, applied properly, they will provide both protection and decoration to virtually any structure.

Failure can usually be avoided through basic preparation and, when necessary, sheltering from poor weather. Simple wetting tests, observation and planning at the outset is also strongly recommended.

If in any doubt contact us.

Solutions to common problems

Shrinkage – as initial shrinkage takes place in the drying out phase, this can be beaten back by using a plasterers’ float and dampening the wall as required – pressing the float home evenly, and in a close circular motion but only if necessary.

Drying out too quickly – Lime renders should never be allowed to dry too quickly, and a render that is simply allowed to dry out too quickly is more than likely to fail.

There is a vast difference between a render that has been allowed to carbonate and one that was simply allowed to “dry out too quickly”.

Pre-wetting the surface

To better control potential shrinkage, we highly recommend pre-wetting the surface to avoid moisture being drawn out of the render coat and into the substrate. Try to avoid over-wetting – pump-up garden sprayers are well suited for this purpose, as a hosepipe will deliver too much water in most cases. In the case of very porous materials such as cob, chalks, and clunch, along with different types of soft brick or stone, the use of a hosepipe may indeed be appropriate.

Weather permitting

You should also pay attention to the weather, as strong sun, wind, frost and rain will all have a bearing on the overall performance of a long-lasting, defect-free lime render.

Work needs to be kept dry enough to allow the lime enough time to set, but do not allow it too dry back too quickly. Try to shield work from direct or wind-driven rain, and where necessary use hessian curtains to stop the work drying out too quickly from wind or strong sun. It is also very important to avoid frosty conditions during the render’s early set, particularly within the first 14 days.

Filling large voids

As lime mortar is more expensive than the stone usually to hand, you can pack out large voids or hollows with a combination of lime mortar and stone.

Scoring / Scratching in

Once the INITIAL set has taken place, key the wall using a convenient tool to make a groove in the render of sufficient depth that will allow the subsequent coat something to grab, or hang on to, without over scoring or tearing the backing coat.

Diamond keying is recommended for scratching in, and a three-pronged lath scratcher is a simple tool to knock up.

Remember: The scoring should not be such as to tear the render off the wall.

Choosing Your Mortar

Rendering on Lath or Render Carriers

First Coat (Bonding Coat)

On any surface one should be looking to apply a uniform thickness of lime render of about 9-12 mm (plasters being the top coat are applied much thinner, 4-7 mm).

For the best results it is recommended to actually ‘throw, cast or harl,’

The material for a cast-on coat should be wetter than that for normal rendering and should incorporate more gritty material. Thrown on by hand, it will provide a suitable bonding coat for the scratch coat. A thrown coat offers a superior bond simply from the action of casting on, and is far less likely to delaminate from the substrate. This is of primary importance on very porous surfaces – such as cob or soft brick – or impervious surfaces such as granite or engineering brick.

Alternatively, apply the first coat as normal using a laying-on trowel, using even pressure to ‘press’ it on or into the wall. Lime mortars are extremely cohesive but require more effort than for cement bound render, requiring greater pressure to press the render onto the surface (aided by the pre-damping).

Application should be reasonably even and once applied should not be overworked or straightened too much. In simple terms, lay it up and leave it.

Note: It is of the utmost importance that an adequate set has taken place in the base coat. To follow on too soon with subsequent coats will result in much greater shrinkage problems, as the individual layers will shrink back at differing rates.

Second Coat

The second coat should be treated the same as the first, and applied before the first coat has developed too much of a set. In normal conditions this should be about one week, but there is no hard and fast rule to the time it may take; Surfaces that are very damp will take longer to harden up. Ultimately, a leather dry consistency is the aim.

The second coat is the straightening coat, so after application the work needs to be ruled/staffed off, to further straighten the work to produce the desired level of finish (if necessary).

Once sufficiently set the render should be rubbed up with a normal float and finished with a devil float to slightly score, forming a key for the topcoat of plaster.

Final Coat

The final coat is treated much the same as the previous coats, assuming any straightening required has been carried out prior to this point.

Once the surface has been laid, avoid rubbing up the work too soon, leaving it for as long as is practically possible.

Top coat plasters will normally have a greater lime content and use a finer sand, so will be more prone to shrinkage problems. Working on lime mortars too soon results in free lime being pulled to the surface (Case Hardening), which affects the properties of the material and can sometimes lead to failure.

The choice of sand in the topcoat is also important dependent as this determines the finish. For a basic smooth finish most BS1200 sands will do, but for work requiring a higher quality finish much finer sand would be required.

Most importantly, the thickness of the final topcoat is crucial and should not be applied any thicker than 5-7mm. Lime plasters supplied by CLC from stock are mixed at 2:3 Lime: Sand, using the most mature lime putty we have in stock.

In Summary

  • Surface preparation needs to be thorough
  • Lime mortars are harder to apply as they need to be drier than that for cement renders, with greater pressure applied
  • Once applied, they require more looking after than a cement render: Keep them damp and protect them from the weather
  • Hair or Fibres must be incorporated when render is going onto a lath carrier

Liability Waiver

The information provided in this guide is intended for general use for operators with limited experience of traditional renders. Individuals will have their own methods and styles and we are not suggesting that tradesmen relearn how to plaster.

The advice offered here is given for guidance only and will assume that best practice will be used in its execution. No claims for liability can be considered on its intent.

Comment On This Article
Ask a Question on our Forum
Get In Touch

Ask a Question on our Forum

45 thoughts on “How to Apply a Three Coat Lime Render

  1. What difference. is there for internal rendering.

    1. Hi Carl

      There is not always a difference for internal and external work. Usually one or two coarse base coats are applied and then a final finish coat. The main difference that we see is within the finishing coat. Internally it is possible to use a finer and thinner plaster coat, such as the CLM66.

      Sometimes the materials will change slightly, for example lime putty is used more internally than say externally. This is especially the case as internal work is not subject to to the elements. However, it is important to allow for internal heating that can have an impact on the curing of the mortar.

      A lot of it comes down to personal preference and how you want the walls to appear.

      If you have any questions, please contact us on 01208 79779 and we can discuss the application in much more detail.

  2. I have lime rendered an external wall with hydraulic lime and sharp sand and would like to add a finish coat as the texture is rough and is very slightly uneven.would it be possible to mix nhl 3.5 with hair and water (no aggregate) and put a very thin coat over it?

  3. hi Laurence i have used lime putty with kiln dried sand to obtain a very fine finish , mixed in a tub with aplaster whisk

  4. gable of church exposed to elements
    scudded with lime mix2/1 plus 6ml grit 1 bucket to each mix
    scud filling out coat mix3/1 plus 6ml grit 1bucket to each mix
    1week between coats lot of rain beating in meantime
    now 2nd coat hasa boaste sound and is friable did not seem to set
    if left for awhile will it eventually set or is this a removeable job
    the finish is one more coat and a harlin coat

    1. Hi Jim

      It is difficult to say for sure without seeing it or knowing the type of lime used. I would say if the render was subject to rain early on and it is sounding hollow and is friable that it may well have to be removed, especially if this is an NHL based render. If the render is putty based it may eventually set, however we are coming into the wrong time of year and the lower temperatures can impact the setting time. I would advise that this coat is removed and reapplied.

      Any questions please contact me.

      Kind Regards

      Adam

  5. I’ve allways used sharp sand for my scratch coat of external render but was wondering can you use building sand? Thinking use that as easier and smoother to put on

    1. Good Morning Si

      The key thing with any sand for use with lime is to ensure that it is clean, washed and well graded. You could use a finer building sand, however we would always recommend a coarser sand for the base coat as it will enable deeper coats to be applied and the render build up should be quicker. We usually find that finer sand can only be applied anywhere from 3 to 7 mm, this is specific to each sand, but usually above these depths you find the render starts to slump. If you are needing to build up the render to around 25mm, this can easily be achieved with 2 coarse coats and a finer finish, e.g. 12mm base coat, 8mm straightening coat and then a 5mm finish. If you are trying to achieve that depth with a finer sand you are potentially looking at additional coats, which will add time and cost. We recommend a 5mm coarse washes sand, such as the CLS28, for any deeper coats and then a 2mm fine washed sand, such as the CLS35, for the finishing coats.

      It will all depend on the specific sand you are looking to use – for example sands in the South West of England are usually very poor, they aren’t washed and can contain a large amount of fines and contaminates. When used with lime this can cause issues within the render and the mortar usually requires much more water than a clean sand and this excess water often results in increased shrinkage.

      The background/substrate will also play a role on the type of sand that is best to use, if the background is relatively smooth you may not require a deep render, so you could use a finer sand for the base coats. However, a thinner render will not offer the same level of protection.

      If you want to discuss the sands and application in more detail please give me a call on 01208 79779.

      With Kind Regards

      Adam

  6. I have just had an external wall lime plastered (three coats). All looked fine for the first 2-3 days after the last coat was added. However, now I am seeing a large number of black spots appearing which are growing in size and are weeping. Tips or reasons for this happening would be most welcome.

    1. Dear Richard

      Apologies for our delayed response, our forum was temporarily disabled.

      It is very difficult to comment on the issues without seeing them and it could be a number of things. Black spots are usually a sign of mould, this could have been present within/on the wall prior to application and due to moisture escaping the render and the wall it could be dragged to the surface. If the spots remain and do not show any signs of damaging the render (i.e. cracking) i would attempt to clean the render with a biocide. Whilst lime is highly alkaline it usually kills of a lot of mould spores, however mould can be stubborn and can grow in almost any condition. Whilst mould is the first thing that comes to mind, it could be a number of reasons, such as leaching from the sand or from the substrate behind. If the problem remains please contact us and we can discuss the issue in more detail.

      Kind Regards

      Adam

  7. Hello

    Do you have any advice or guidance on using lime with micafill to create an insulating render for an external stone wall? The wall is drystone, but I have filled the voids and rough pointed with NHL5/sand. All the stones are well smeared. Was intending to spray the render on with a splatter gun. For cement, they recommend 25kg per 100litre. Would the same work with lime?

    Any help / comments welcome!

    Thanks

    Simon

    1. Hi Simon

      In terms of micafil or vermiculite it is not something that we recommend using with lime. Vermiculite is often used in lightweight renders as it is cheap, however it does have some technical issues – it has a two-dimensional structure, so w applying the render it can breakdown and reduce both the durability and insulating value of the render. In cases the vermiculite crushes and this leaves an extremely weak render and offers very little insulation value. It is also requires a lot more water to be added to the render and this can lead to slumping of the render. A lot of the premixed lime renders using vermiculite either have another material or cement added to make them work.

      We do produce a premixed Insulating Render (http://cornishlime.co.uk/cornerstone-insulating-render) which does not use vermiculite, if you would like any more information on this please contact me.

      Kind Regards

      Adam Brown

  8. Hi, If I’m rendering with NHL 3.5 do I need to add any lime putty in the mix or is the NHL ok without the putty.
    Thanks Gary

    1. Hi Gary

      Using NHL3.5 without putty is fine, small amounts of putty are sometimes added to try and help improve the workability, but this is usually personal choice of the applicator.

      Kind Regards

      Adam

  9. Hi…doing a wall at the moment…its been scudd edd with a 2:1 mix of sand and cement…is this ok to use nhl 3.5 oher it?

    1. Hi Jimmy

      Providing there are no waterproofers added to the sand and cement mix, NHL3.5 and a suitable sand will be fine to go over this.

      Kind Regards

      Adam.

  10. Hi. I am attempting to render the walls inside my house using NHL 3.5. I had to take off old cement render which was preventing the walls to breathe. The exposed bricks just seem to endlessly absorb water & dry out quickly. Am I best keeping an eye on the newly rendered wall by routinely spraying each coat so not to risk the render drying out too quickly or am I doing it wrong? Thanks

    1. Hi Lee

      Older bricks are very often porous and it can be difficult to achieve the correct moisture content in order to prevent the render from drying to quickly. Often before rendering we advise to saturate the wall and allow it to begin to dry back and then spray water when/if required, this may take a little time, but it is easier to control the suction of the wall. It also may help if you were to apply a slurry coat of lime, sand and water to the walls and cast/throw on a base coat of render when the slurry is still tacky, the slurry can help control the suction from the brick and the casting/throwing action ensures a good bond between the substrate and the render.

      If you have already applied the render it will likely be a case of monitoring and spraying the walls with water repeatedly to prevent the render from drying to quickly, this will be time consuming but if the render is already applied is the only real way to keep the wall damp. It is also advisable that any heating is turned off.

      Kind Regards

      Adam

      1. Thanks Adam
        I didn’t go with the slurry, I ended up saturating the affected walls & applied a scratch coat.
        Thanks for your help.

  11. This is an amazingly thoughtful and well presented page. Thanks for producing. Thanks too, for spending so much time on replying to the individual questions. We’re a few months away from beginning the process of plastering. Your comments and advice give me a lot of confidence to take on the lime re-plastering of our renovation project.

    1. Hi Roy

      Thank you for your kind comments. When it comes to starting your project if you have any questions please do not hesitate to contact us and we can discuss the works in more detail.

      Kind Regards

      Adam

  12. After applying base coat with harling trowel do you then use plastering trowel to flatten the finish a bit and scratch with diamond shapes or just leave as is for float coat , Regards Ian.

    1. Hi Ian

      If the harled coat is rough enough there is no need to apply a trowel over the surface, however it will depend on the number of coats that you are looking to apply. We usually advise that the first coat is cast/thrown onto the wall as a bonding coat at around 4 to 7mm and this is left as a rough finish. A straightening coat can then be applied to this with a trowel and ruled/staffed off to achieve a flatter finish, this can then be scratched and a finish can be applied over this.

      If you did want to apply a trowel over the first coat or apply the first coat with a trowel this would be fine, but is not always required.

      If you have any questions please contact us.

      Kind Regards

      Adam

  13. Hi,
    I have an internal cob wall with lime render and it needs replacing – it is also on a curved wall which i want to keep. The builders I have in at the moment have never plastered/re-plastered/done any lime rendering and have concerns as how to progress. They will need to take off the old lime render but are wondering what/how they apply straight to the cob , which is soil and straw, amongst other things! I believe they cannot use plasterboard. The external wall of the cob is brick and normal paint.
    Any suggestions how they could progress would be very helpful.
    Thanks

    1. Hi Catherine

      If they are removing the existing render the condition of the cob will determine how you move forward. If the cob is damaged this will need to be suitably repaired with either mortar, cob block or ties (impossible to say without seeing it). If the cob is in good condition or when it has been repaired we would only recommend using a lime putty or NHL2 mortar for the works, as cob is a weak background only low strength limes can be used. Dampening and controlling the suction of the cob will be critical, cob is usually able to hold significant amounts of moisture and the cob will need to be dampened down sufficiently before any work is carried out. If the cob is not sufficiently dampened it may take water/moisture from the render and this can result in shrinkage, a poor bond or failure of the mortar. The mortar should be thrown onto the wall or cast, this impact from throwing achieves a good bond between the mortar and the cob. This coat would then be left rough and left to cure, a second coat can then be trowel applied, this would then be staffed or ruled off to a level finish. Once this coat has sufficiently set, a fine lime plaster, such as our CLM66, can be applied over this coat at around 2mm.

      The above is a brief overview, but if you would like to contact us we can discuss suitable materials and application methods, if you can send any photos that will also help us better advise on the best way forward. Cob can be more demanding than other materials such as stone, and the contractors will need to take care and ensure the cob is sufficiently dampened, that the correct strength of lime is used and time is allowed between each coat. If they wanted to contact us before or during the application we can also discuss the work in more detail.

      With Kind Regards

      Adam

  14. Hi Adam

    I have completed a lime render project maybe 3/4 months back now following the guildlines provided,three coats first two coats with horse hair and course sand final coat with fine sand using nhl 3.5 thoughout and with some trial and error got it to a decent standard!however the final panel being the front of the house appeals not to be drying out it has been painted with a dark green lime wash but has a lot of dark patches on it!?it does seem worse along the bottom where I did notice upon application No real mechanical key very low suction witch would surgest damp bricks?but there is all so smaller patches higher up!can you advise me what this might be

    1. Hi Chris

      It is very difficult to answer without seeing the panel and the issues and without knowing the NHL brand, sand quality, lime wash and other various factors such as weather conditions, drying times etc., if you can send any photos and additional information over I may be able to better advise on the issues.

      Below are some initial thoughts/questions as to why this could be happening –

      At the bottom of the wall does the render come into contact with the ground at all? If it does this could suggest that any water/moisture on the ground is being absorbed into the render and could suggest the darker patches at this lower section. Also what is the ground finish near the render? Is it a hard standing/impermeable finish, such as concrete or paving? If the ground covering is impervious any rain could be thrown back into the render and increase the moisture levels in this section.

      Are there any signs of damage to the render? hairline cracking or larger cracks? Any surface growth, such as efflorescence?

      It could be as simple as areas where excess moisture is present within the background and will just need time to dry back naturally. Limewash is an absorbent paint finish and relies on being built up over numerous layers to provide protection, if there is high levels of moisture and/or rainfall the render and paint will change colour as water is absorbed and released. Limewash will often look patchy and change colour as it goes through wetting and drying cycles and it could be the render and paint naturally drying.

      As i mentioned earlier if you can send over some pictures of the areas in question i can better advise.

      Kind Regards

      Adam

  15. Hi
    just planning to lime render a renovation project the walls are brick so we are lime rendering for the aesthetics as the location is rural we have to insulate the house from the outside.

    Its a mine field out there with information on what’s best to apply lime on to board wise so we are wondering what would the lime render has to be applied over we’ve been told wood wool boards but we are trying to see if there’s a cheaper alternative.

    any help advise would be greatly appreciated

    kindest regards

    Carl

    1. Hi Carl

      There are a number of systems on the market and it can be quite difficult to know which one is best suited. That said there are a number of alternative options when it comes to insulating solid walls with lime render and some can offer a significant cost saving.

      Do you have specific U Value or targets which need to be achieved? Or are you simply looking to improve the thermal performance of the walls?

      If you could email me any information you have on the property and any targets/value or conditions which have to be achieved, I can put together some information on various options and costings for you. My email is adam@cornishlime.co.uk or alternatively you can contact me on 01208 79779.

      Kind Regards

      Adam Brown

  16. Hi
    We have an old georgian house with original lime plaster walls.
    It was near derelict when we moved in with wall paper hanging off the walls etc, so we stripped it off and just painted the walls and ceilings with builders trade emulsion paint as a cheap solution to make it look acceptable, until we could renovate.
    We were told this would be fine, as long as it wasn’t latex paint.
    Now I want to get some ceiling and walls reskimmed with lime, but someone on the internet said we won’t be able to now as it’s been painted.
    Is that correct?

    1. Hi Kate

      Unfortunately you wont be able to skim directly over the paint. Lime plaster will need an absorbent and porous background for it to be able to bind and adhere. There are primers on the market which can be painted on and then a plaster applied over the top, but these will significantly reduce or prevent vapour permeability. We would always advise against plastering over paints, as it can lead to issues long term and you are relying on the paint to hold as long as the plaster.

      So it would be a case of trying to remove the existing paint.

      If you have any questions or would like to discuss the application in more detail, please contact us.

      Kind Regards

      Adam

  17. Hi,

    I’m planning to plaster the downstairs walls in my Victorian end terrace with lime plaster. There are some damp issues in the corners and I hope that by removing the old plaster and putting on a breathable plaster painted with limewash will help to remedy this.

    I am trying to decide between using NHL 2 or lime putty. I’ve used NHL 3.5 to plaster a wall previously and thought it was straightforward to work with, but would be interested in using lime putty to make the wall as breathable as possible.

    My main concern is the time it will take for the plaster to cure between layers and the additional workload of ensuring the plaster doesn’t dry out too quickly and dealing with shrinkage.

    Do you have any experience of using lime putty plaster on internal walls and any rough idea of how long to wait between coats. I know it can vary significantly. Do you have any opinions of the suitability of NHL 2 vs lime putty for internal plaster? I would like to take the easier option of using NHL, but if lime putty is necessary to help control damp then I’m happy to make the extra effort to use that.

    Thank you very much for all the articles and information on this site. It’s really useful.

    1. Good Morning

      Either option will be suitable, lime putty will offer a higher vapour permeability but the NHL2 option still offers a very high vapour permeability so there is likely little difference between the two options. Both will be prone to shrinkage and require aftercare, the putty option may need a little more care but a lot of this is dependant on the application, moisture in the background etc. If you have used NHL before and had a good experience with it, it would be fine to use NHL in this application. The drying times on the putty may also be slightly longer, again this is variable, but with putty you may be looking at say 5 to 7 days between coats and with the NHL2 it may be 3 to 5 days. The NHL2 can be site mixed with appropriate sands or we can supply the material premixed, the premixed option is under our Cornerstone range adn has additives which can help with shrinkage. If you are looking for a fine plaster, we would suggest using lime putty for this as it will be easier, our CLM66 is regularly used for top coat plasters.

      If you have any questions or need any further information please do not hesitate to contact us.

      Kind Regards

      Adam

      1. Thank you for your reply. I’ve decided to go with lime putty and have bought a couple of tubs of your Cornish Lime Putty from builder’s merchant in Bristol wo you supply. I’m considering adding hemp fibre to sharp sand base coat to increase insulation. Do you have any guidance on the proportion of hemp fibre to add?

        Thanks so much for your help.

        Adam

        1. Hi Adam

          Adding hemp fibre wont do much to improve the thermal performance of the mortar, if you are using as a replacement or in the same way as adding hair or polypropylene fibre. If insulation is a factor we produce a premixed, lightweight insulating render that can be used as a base coat, under our Cornerstone range and this offers very good insulating properties whilst performing the same way as an NHL2 mortar.

          To improve the thermal performance through adding hemp it would need to be a significant addition and this would be replacing sand within the mix, for which you would need a hemp shiv rather than a fibre. One issue with hemp shiv and lime putty (also some NHL) is the drying time is increased quite a lot, as the hemp shiv will cling on to moisture keeping the coat damp. If you are going down this route you would need to allow for this longer curing process. We have experimented with hemp renders and found drying times of 6 weeks plus, unfortunately I am not sure of the addition rates of hemp though.

          Kind Regards

          Adam

  18. Good Afternoon,

    I’ve been repointing the front of our house with NHL 3.5 and CLS17, but need to do a small amount of rendering on the side. I see the lime is used in above comments, but is the sand appropriate? Also are all three coats of render a 2:3 lime:sand mix or do they differ for each coat? And lastly can you give any guidance on the amount of water to add?

    Thank you for all your advice and guidance,

    Francis

    1. Hi Francis

      We wouldn’t recommend using our CLS17 for rendering, the reason for this is that the sand is very dusty and contains a high fines content, when mixing with an NHL you will need to add quite a lot of water to make the mortar workable and this increases the risk of shrinkage and cracking. We would recommend using our CLS28 or CLS30, these are coarse washed sands and as they are washed a lot of the dirt and dust is removed and it is graded to have a lower fines content, making it less prone to shrinkage and cracking. These sands are generally used for base coats or building out the render and can be applied up to 15mm per coat. If you are looking to achieve a finer/thinner finish on the render, we would advise using our CLS35, this has a similar profile to the 28/30, but has the aggregate removed so can be applied from 5 to 7mm and will offer a smoother finish.

      The mix ratio of 2:3 is for our premixed internal finishing plaster and we wouldn’t recommend this mix ratio when working with NHLs or for external applications. The mix ratios will depend on the number of coats which you are looking to apply, but the key thing with any render application is you must get weaker as you build the render out. For base coats we would advise using a mix ratio of 2 parts lime to 5 parts sand, measured by volume, with the first coat being the thickest and any subsequent coats being applied thinner. If you are applying a smoother finish coat with the CLS35, we would advise using a mix ratio of 1 part lime to 3 parts sand, again measured by volume.

      The drying time between coats will vary and can depend on several factors such as time of year, moisture in the background, previous coat thickness. As a general guide we would say allow 4 to 7 days between each coat and ensure previous coats are kept damp for the first few days so they don’t dry too quickly. Also the surface will need to be dampened before applying the next coat, this will help control the suction of the background.

      Water addition is difficult and again varies depending on the moisture content of the sand and also how stiff or dry you would like the material to be. We would advise using the least amount of water as possible, as more water increases the risk of shrinkage. If you slowly add water to the mix it will be easier to control, its also advised that once the mortar has been mixed to let it stand for around 20 minutes, this quenches the mix and makes for a nicer product to use, you can also then knock the mortar back up and add anymore water if needed.

      An alternative to site mixing could to use the Cornerstone mortars, these are dry, pre-blended NHL mortars with the correct mix ratios for each application and will use more of a constant water addition. There are also some additives, which are conservation approved, to help mitigate the risk of shrinkage and improve the bond with the wall.

      For further guidance on render, we would recommend our three coat render application guidance, which can be found here – http://cornishlime.co.uk/information/appy-three-coat-lime-render/

      If you have any questions, please do not hesitate to contact us.

      Kind Regards

      Adam

      1. Thank you very much Adam for the detail. That is brilliant.

  19. Hi Adam
    What is the correct mix ratio for NHL 3.5 and sand for re-pointing and render internal walls? I have cleared the old mortar, dampened the walls, etc. I have been advised to use 3 parts lime to 5 parts sand. Is this correct? How much water should I add? And what consistency am i looking to achieve ie: should it be quite tacky and stick to a trowel if its held upside down, should it be grainy and fall off a trowel? Also, how long do i need to keep dampening the walls after re-pointing and rendering? Really appreciate any advice you can give me.
    Many thanks
    Jeana xxx

    1. Hi Jeana

      The mix ratios may vary depending on what the mortar is used for, for example for repointing you could use a ratio of 1 part lime to 3 parts sand or 2 parts lime to 5 parts sand, for rendering your base coats could be 2 parts lime to 5 parts sand and then for a finish coat you could go with 1 part lime to 3 parts sand (always mixed by volume). 3 to 5 would be too strong in our opinion.

      In terms of consistency, we would recommend a stiffer mix for pointing and a wetter mix for the rendering, but this also comes down to personal preference.

      We would advise that the mortar is kept damp for at least 3 to 5 days after application/between coats.

      If you have any other questions, please do not hesitate to contact us.

      Kind Regards

      Adam

  20. Hi Adam,

    ..a wealth of information on this site, thanks.

    I’m currently renovating an old cottage and have pointed most of the walls internally. There are however a couple of walls that I plan to render..I’m looking for quite a ‘rustic’ look… which should be easy given my lack of skills 😉
    With that in mind, would 2 coats suffice? First coat harled and then a roughish finish coat?

    Thanks,
    Ciaran

    1. Hi Ciaran

      The undulations of the wall and then the desired finish will ultimately determine the number of coats needed. Three coats is a common application, but this is not a hard rule, the key is ensuring each subsequent coat becomes thinner and weaker as you build out. It is worth noting that more coats provides greater durability, but internally this is not going to be such an issue. If your walls are relatively flat you could possibly get away with a single coat, but a rough finish should be achievable exactly how you have mentioned.

      Good luck and any questions please contact us.

      Kind Regards

      Adam

      1. That’s great Adam, thanks for the advice!

  21. When applying the splatter coat before the base coat what materials ie sand lime and what aggregate to achieve a coat good enough to accept the base coat. Also what ratio do you mix lime and sand for external render.am doing up a house in the north of England in late October or early November would you recommend lime rendering it then or wait till next year thank you

    1. The splatter coat will need to be a mix of coarse sharp washed sand, and the same lime you’re using for the basecoat. You will need to use a stronger mix ratio then the basecoat, for example if your basecoat is 2:5 then your splatter coat should be 1:2 (lime : sand). In terms of mix ratios I’s suggest 1:2 for a splatter coat, 2:5 for a basecoat and 2:5 for a levelling coat, the levelling coat needs to be thinner than your basecoat if you maintain the same mix ratio.

      I would advise leaving rendering now until next year, it’s too late in the year and a fresh render may not develop enough strength to resist the winter if the weather is particularly cold this year. Doing the work in spring gives a fresh render the best chance to succeed.

      Kind Regards,

      Edd

Leave a Reply

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.