Burnt Sand Mastic

Suitable for internal and external use on metal or timber window and door frames. Cornerstone Burnt Sand Mastic should not be used to repair damaged rises or defective renders. Always ensure any repair work required is adequately completed before any mastic work begins.

Cornerstone Burnt Sand Mastic is supplied in two parts, including 16kg of burnt sand and 2.5L of oil driers as an oil blend. This provides a volumetric mix of 4 parts powder to 1 part oil, the standard mix ratio.

Preparation
Background preparation will vary depending on the substrate and purpose for application. Generally, always ensure surfaces are clean and free of dust and other debris, and that best practice is followed. Coverage will vary depending on the joint depth and size. Apply to a test area to estimate the coverage relevant to your project.

Before pointing, ensure that all wide gaps between the frames and masonry are tightly packed with a fibred or well-haired lime mortar that has cured and hardened. Keep the joints slightly recessed 4-5mm. Take a small painter’s fitch brush and apply a light primer coat of the oil dryers to the frame and adjoining masonry surface, ensuring the line of the proposed mastic joints is maintained. View or range of lime mortars.

Mixing
Mix only the amount which can be used within a few hours. Ensure to mix thoroughly using the minimum amount of oil. For the best colour results, mix the whole tub all in one go. See the datasheet for detailed mixing guidance.

Application
Cornerstone Burnt Sand Mastic can be applied with several tools, which is most appropriate and will vary depending on joint width and depth. In general, a mixing pot, a small hawk or mastic box, a small tool or proprietary mastic trowel and a painter’s fitch brush are options worth considering. Always wipe your tools with an oiled cloth during the application and use a cleaning solution to clean them after application.

Apply Cornerstone Mastic firmly into the angle of the frame or jamb, filling the bottom up to form a joint to a neat 45-degree angle.

After forming the angle, starting from a corner, press a clean, oiled mastic trowel tightly against the face of the timber frame and masonry jamb and carefully press and draw the trowel to create a neat regular fillet. Clean away any excess mastic and wipe the finished timber edges on completion.

For mastic to sills, ensure the mastic does not bridge any drips and is left recessed behind the drip check. Mastic fillets should be neat, 20 – 25mm across the diagonal face.

Aftercare
Aftercare will require time for the mastic to cure and harden fully. The time required for this process will vary depending on environmental temperature, generally between 1 and 3 weeks. We always advise following best practices for curing and protecting fresh burnt sand mastic from direct sunlight and wind using hessian sheeting or plastic sheeting, which acts as a barrier between the freshly applied mastic and the environment.

Any materials with oil present, such as paper towels or cloths should be dunked in water before disposal, as there is a small risk of fire in warm conditions. See the datasheet for information on health and safety data relating to oil.

See the Technical Data and Documentation section for detailed product information, application guidance, and datasheets. Always review any available documents before use. Each project is unique, always seek professional advice before undertaking a project.