Roundtower Burnt Sand Mastic is a traditional, oil-based jointing compound designed for creating durable, flexible joints between timber or metal window and door frames and surrounding lime render, plaster, or masonry. Formulated with a blend of burnt sand, natural oils, and driers, this mastic cures slowly while remaining flexible—accommodating minor movement and helping to prevent cracking, moisture ingress, and internal damp.
Used extensively in conservation and restoration projects, Roundtower Burnt Sand Mastic provides a weather-tight, breathable seal and is compatible with solid wall buildings and lime-based finishes.
✔️ Ideal For: Timber or metal frames adjoining lime render or plaster
✔️ Weather-Resistant: Creates a sealed, water-tight joint for long-term performance
✔️ Flexible & Breathable: Accommodates movement while maintaining vapour permeability
✔️ Textured Finish: Available in 12 standard colours; custom colours on request
✔️ Standard Mix Ratio: 4 parts burnt sand to 1 part oil blend (supplied as 16kg + 2.5L kit)
✔️ Application: Suitable for both internal and external use
Product Features
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Traditional formulation using burnt sand and linseed oil
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Helps prevent cracking, damp, and mould in reveals
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Easy to mix and apply using standard mastic tools and painter’s brushes
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Ideal for forming neat 45° fillets between frames and masonry
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Not suitable for repairing damaged rises or defective renders—substrate repairs must be completed first
Application & Mixing Guidance
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Prepare Background: Surfaces must be clean, dry, and dust-free. Pack wide gaps with well-haired lime mortar, cured and recessed by 4–5mm.
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Prime: Lightly coat the frame and adjoining masonry with the supplied oil drier using a fitch brush.
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Mix: Combine only the amount needed for immediate use. For best colour consistency, mix the entire tub at once.
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Apply: Use a hawk, mastic box, or trowel to firmly bed mastic into the joint. Form a 45° angled fillet and smooth with an oiled trowel.
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Detail: Ensure no bridging occurs over drips or sill features. Clean excess immediately.
Aftercare & Curing
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Allow 1 to 3 weeks for full cure depending on temperature and humidity
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Protect fresh mastic with hessian or plastic sheeting to prevent rapid drying
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Dispose of oil-soaked materials carefully—dunk cloths in water before disposal to avoid spontaneous combustion risk in warm conditions
Roundtower Burnt Sand Mastic offers a sympathetic and robust solution for jointing traditional building elements while supporting the health and longevity of solid wall constructions. It is part of the Roundtower range of breathable fillers, mastics, and mineral paints—trusted by professionals across heritage and conservation sectors.
