Burnt Sand Mastic
Traditional joining compound for forming joints between timber window and door frames and render or masonry. Burnt Sand Mastic hardens slowly but remains flexible to reduce the risk of cracking.
Burnt sand mastic is supplied in two parts. Burnt Sand + Oil-Driers. It adheres tenaciously to most surfaces.
Area of Use
For use where timber joins render or masonry.
The Burnt sand mastic is combined with a mixture of oils and driers (supplied with the sand mastic) on a clean surface, mix thoroughly using the minimum amount of oil. (If the mix is over wet, let it stand in a warm place for a few hours to stiffen up then remix vigorously before use).
Once mixed with oil driers, mastic will harden. Mix only that which can be used in a few hours.
Before pointing ensure that all wide gaps between the frames and the masonry are tightly packed with well haired lime mortar that has been allowed to cure and harden. (This was the original material used to bed window frames). In some circumstances inert filler boards of suitable packing can be used. Keep the joints slightly recessed 4-5mm. Take a small painters fitch brush and apply a light primer coat of the supplied oil dryers to the frame and the adjoining masonry surface, making sure that the line of the proposed mastic joint is maintained. Do not over oil surfaces.
Working with a mastic box or a small hawk and a proprietary mastic trowel press the mastic firmly into the angle of the frame / jamb and filling from the bottom up, forming the mastic joint to a neat 45 degree angle. Keep tools wiped with an oiled cloth during the application. 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 should not be over painted. Mastic fillets should be neat 20 - 25mm across the diagonal face, mastic should not be used to make up damaged arises or defective renders, these deficiencies should be repaired properly before mastic work commences.
Each project is unique, always seek professional advice before undertaking a project.