Technical Advice Note for Bag Rub Finish

Bag rubbing is a simple technique where mortar is applied over masonry in such a way that its neither pointing or rendering, yet offers a sacrificial and protective coating to a wall, without having to extinguish its underlying character.


Before getting started, remember that the appropriate PPE measures should always be used when working with lime. While the joints to the masonry need to be appropriately prepared it doesn’t necessarily demand the same thoroughness as that for pointing. Nonetheless the works should be conducted in a diligent manner with the joints cleaned out to a depth not less than 10 mm but ideally 10-15mm for improved key.

The joints are then brush cleaned to remove excess dust and depending on conditions careful washing out with a hose may be appropriate; however, this would preferably be done at least the day before the application of mortar.

Prior to application the background – masonry & mortar joints should be adequately dampened with a fine mist spray (or similar) to control suction and prevent rapid moisture loss from the fresh mortar. This is critical to the overall performance of the works. For more information on this important aspect of the work, see our Guide to Curing Lime Based Renders and Mortars.


Assuming that all repairs and other preparatory works to the masonry have been carried out and that appropriate lime and sand for the mortar have been selected, the mortar should be prepared and mixed accordingly. It should be applied as a single coat application, preferably with the mortar thrown onto the masonry and then thoroughly pressed into the joint recesses. Once ready and while the mortar is still in a plastic state it should then be rubbed up. This is best carried out using a circular scouring action similar to that of using a plastering float. Hessian sacking is generally the material of choice for this operation but other types of suitable material may be used, the purpose is to scour it back firmly compressing the mortar into the joints between masonry units and into any fissures in the stone.

After the initial “set” has taken place and as the mortar is “picking up,” while not essential it would be beneficial for the mortar to be rubbed over again; which helps to close up any initial shrinkage. This should be done using either a damp sponge or the hessian, feeding an appropriate amount of water from a mist sprayer or similar onto the surface to aid hydration/carbonation.

Bag rubbing is NOT meant to be a render, in this application any excess of mortar over and above that needed to fill the joint is rubbed over the entire surface of the masonry as a smear coating; an additional benefit of a bag rub finish especially on softer friable stone, is that it works as a surface consolidant acting as a filler in any cracks and fissures in the stone itself as well as providing a sacrificial “shelter coat” over the surface.

Bag rub next to a window frame.

Curing and Aftercare

Most importantly and regardless of the lime binder used, the mortar should NOT be allowed to dry out too quickly during its early stages of curing! Depending on conditions at the time of application we stress that adequate measures and provisions are in place or available to implement appropriate curing and protection.

After placing, lime mortars need to be properly cared for; during the winter, they need to be protected from too much rain, especially during any spells of frost which can damage the mortar or cause it to fail completely. At all other times mortars should be prevented from rapid drying, either by wind or sun as this will desiccate (dry out) the mortar and prevent proper hydration or carbonation of the lime binder used, often resulting in a failure whereby the mortar becomes very weak and friable. Therefore, appropriate protection is a must. In all instances it may be beneficial to protect the hessian with polythene; during the winter to keep it dry so improving its thermal protection, and during the summer it will help to keep it damp for longer, increasing humidity where it’s needed.


Bag rubbing is a very simple and cost effective technique that can be subsequently painted (not essential) with either a limewash or the Beeck’s Mineral Paints. See our full range of breathable paints. The overall appearance of the stonework needn’t be lost, resulting in a surface that has been unified and tidied up at a fraction of the cost of pointing or rendering, making it an extremely cost effective and somewhat traditional finish to walls that were very often meant to be covered in any case. This approach does not require a high level of skill to execute, but care and consideration will determine the outcome.

Close up detail of a bag rub.
An example of a bag rub

The Mortar

The mortar should be made using sand appropriate to the masonry being worked on. A 50/50 sand such as CLS30 (50/50 coarse and fine sand – supplied by Cornish Lime) is the most popular for this type of work; while CLS35 (a fine clean washed plastering sand) has been used to good effect on numerous projects, where a coarse sand wasn’t appropriate to the masonry. View our range of coarse and fine sands.

It is important to select a lime that will yield a mortar strength weaker than the host masonry and our suggested mix would be at 2:5 lime to sand.

These mortars can be supplied in various forms; separate materials (lime & sand) for site mixing, premixed putty mortars or as dry premixed NHL mortars. View our full range of Cornerstone lime mortars.

10 thoughts on “Technical Advice Note for Bag Rub Finish”

    • Hi Richard

      The mortar can be applied by spray method, the hopper style sprayers with a compressor will work, such as the Sablon rendergun.

      If using a sprayer you would need to take more care as you would need to limit the amount of mortar that is delivered to the wall. A spray method could cause a build up/depth of mortar that is too great for a bag rub finish. We usually suggest aiming for anywhere from 5 to 10 mm depending on the substrate and desired finish. The mortar would still need to be rubbed back to follow the contours of the stone.

      So whilst it can be sprayed and we have seen it carried out this way, the sprayers are used more commonly for base coat renders.

      Any questions please contact us.

      Kind Regards

      Adam Brown.

    • Thanks for your enquiry. Yes, this should be achievable but we couldn’t say for sure without seeing an image. If you wanted to forward one through to us we we would gladly take a look for you.

  1. Do gaps in the masonry (no or minimal mortar) need to be filled prior to lime-bagging or is it okay to add extra amount of lime/sand mix to those areas to fill them as you lime-bag the whole wall? Thanks

    • Thanks for getting in touch. It all depends on the size and depth of the gaps – we can’t really say as it will be specific to each application. Anything relatively deep, i.e. 10mm depth or greater or 4mm width or wider may need to be filled in beforehand.
      Please let us know if we can be of further help.

  2. Really excellent article about a technique that should be used much more. It’s a modest, economical finish that is ideal for new lime users and DIY.

    • Hi Jane, thanks for your comments – we agree that it’s a super finish and have a few case studies on the site to showcase properties that have been bag-rubbed.

    • Thanks for getting in touch. Yes, you can use the bag-rub method internally but we’d recommend painting afterwards with something like our Beeck Maxil Pro as it’s wipeable which you’d need in that kind of area. Please get in touch if we can be of further help.


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