Aberdeen is known as the Granite City. Glasgow’s tenements are largely red and blond sandstone, and Edinburgh’s Old and New Towns are a UNESCO World Heritage Site built in stone. Across the South East, too, many older homes have stone sills, surrounds or walls. Natural stone behaves differently from brick, render or uPVC, and the wrong sealant can stain it, fail to bond or trap water. This guide explains what to look for.
How stone differs from other surfaces
Sandstone is porous. It absorbs water and can hold it. Its surface can be soft and dusty, so sealant may grip loose grains rather than sound stone.
Granite is hard and dense, but many granites still have a slightly porous surface, and polished granite can be smooth enough to make adhesion harder.
Both can be stained by some sealants. Ingredients in certain sealants can migrate into porous stone and leave a dark band along the joint that can’t be cleaned off.
Choosing a sealant for stone
Look for a sealant the manufacturer specifically lists for natural stone. Adshead Ratcliffe’s Arbosil XL1099, for example, is described as a one-part non-staining silicone for joints in natural stone such as granite and marble, and the manufacturer strongly recommends testing it on the stone before use (Arbosil XL1099 product data).
By contrast, some general window silicones are not recommended against natural stone. That is why reading the data sheet matters. It is also why acetoxy (vinegar-smelling) silicones are generally avoided on stone and metal, as they can attack some surfaces.
Traditional buildings and sand mastic
On older stone buildings, particularly around sash and case windows, the traditional joint filler is burnt sand mastic: a linseed-based mix with sand that is pointed into the joint. It suits the look and the way traditional stone buildings handle moisture. Our guide to sand mastic for sash and case windows explains it in more detail, and our sand mastic service covers this work.
If your building is listed or in a conservation area, check with your local council before changing the type or appearance of materials on the outside.
Need it done properly? Mr Mastic covers the South East, London and Scotland, with a one-year guarantee on every job. Call or email for a quick quote.
Preparing a stone joint
- Cut out all of the old mastic or sealant, without damaging the stone edges.
- Brush out loose grains and dust so the sealant meets sound stone.
- Make sure the stone is dry. Sandstone can stay damp inside long after the surface looks dry.
- Use a primer where the sealant manufacturer calls for one.
- Use a backing rod in deep joints so the sealant is the right depth.
- Test on a hidden area first if staining is a concern.
Where the work is
We cover all of Scotland, as well as London and the South East. On stone homes, that usually means window and door perimeters, stone sills and the joints where newer frames meet old masonry. Our window and door mastic service covers modern frames in stone openings. Every job carries our one-year guarantee.
Why stone joints fail
Stone joints usually fail for one of a few reasons. The sealant bonded to dust or loose grains rather than sound stone. The stone was damp when the sealant went on. The joint was too shallow for the sealant to stretch. Or the product was never suited to stone in the first place. Each of these can be avoided with good preparation and the right product.
Quick answers
Why is there a dark line along my stone sill?
It may be staining from a sealant that has migrated into the stone, or trapped damp. It is worth having looked at before resealing.
Can silicone be used on sandstone?
Yes, if it is a product the manufacturer lists for natural stone and the joint is properly prepared. Testing on a hidden area first is wise.
Is sand mastic only for listed buildings?
No. It suits many traditional stone buildings, listed or not, particularly around sash and case windows.
Related services
More advice
- Sand Mastic for Sash and Case Windows
- Scotland’s Weather and External Mastic
- Window Mastic: When to Replace It and How
- All advice
This guide is general information about sealant work. It isn’t legal, fire-safety or structural advice; for those, speak to a qualified professional.