Supplied in two parts. Burnt Sand + Oil-Driers. Used to form joints between timber window and door frames and masonry or rendered masonry, traditional burnt sand mastic has been in use for at least 200 years. It hardens slowly but remains sufficiently flexible for the purpose. It adheres tenaciously to most surfaces.
It is made by roasting sand on a hot plate or in a small kiln and was originally mixed with oils, lead based driers and crushed chalk. However for many years now, alternative natural driers have replaced the lead products.
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.
We supply stone repair materials and other lime products for repairing renders
Note: Mastic should not be over painted.
- Technical Documents
- Hardens slowly but remains sufficiently flexible for the purpose.
- Adheres tenaciously to most surfaces
Preparing the joint
New installations should follow traditional practice. Repair work may have to take account of erosion of masonry, movement, poorly fitted previous replacement windows or doors or poorly executed repair work including non hardening modern mastics which require replacement.
Eroded, defective masonry should be repaired to form a neat edge where practicable to maintain the burnt san mastic joint at a consistent size (approximately 20mm). Burnt sand mastic is not a masonry repair material: Joints that have lost their lime backing and where there is clear space between the frame and the masonry must be packed, with well haired lime mortar. In all events the gap must be filled to within 10mm of the plane of the ingo.
Sash and case windows have weight pockets and when filling the gap, care should be taken to ensure nothing enters the weight pocket and affects the free running of the window. For this reason expanding foam should be avoided.
The timber frame should have loose flaking paint removed and be fully primed and undercoated before applying the finished mastic. Check the absorbency of the masonry, repair or render with water as the mastic is oil bound and high absorbency may occasionally result in bleeding from the mastic into the surrounding masonry.
If the masonry has a high surface absorbency, use tape to protect and mask the masonry to prevent undue surface spread. (Trial samples should be carried out) Although oil stains usually do evaporate over time, avoidance is the best option.
Using a small painters fitch, neatly apply a small quantity of the oil drier directly onto the frame and masonry to prime the surface ready to receive the mastic. Keep the primer to the line of the mastic avoid over priming. This can be done up to 1 hour before applying the mastic (weather dependant). Do not allow the oil primer to dry, apply the mastic while it is still feels oily or tacky to the touch.
As supplied Burnt Sand Mastic is a two part product, the burnt sand, in a tub and the two part oil and drier in a plastic bottle. For each litre of burnt sand add approximately 200ml total of oil hardener is added a little at a time, mixing carefully at each addition until the mastic is worked into a thick putty like consistency and comes cleanly of the mixing trowel. It should be possible to turn virtually all the material into a large ball with a single flip or the trowel. Allow to stand in a covered container for approximately 2 hours prior to use to ensure that all the oil and hardener has fully integrated with the burnt sand. Remix immediately before application.
- Oil Drier: Oils containing a drying agent that will cause hardening over time
- Mastic Trowel: Long rectangular trowel square sides approximately 18-20mm wide.
- Mastic Box: Open ended three sided wooden box with handle to hold mastic while working 150mm x150mm x 50mm deep (a harling trowel can also be used)
- Ingo: Inband return from face of wall to window or door frame
- Fitch: Thin angled paint brush used for cutting in angles and corners