filling pitted window sills

[alan]

I'm currently stripping off 20 layers of paint from my 'sandstone' window sill to reveal heavy pitting as a result of 100+ years of weathering. In a few places the surface is still flat and hard but in others the pitting is 6? by 4? and rubbing it with a finger produces lots of sandy dust. In the past the leading square edges have been sanded down to tidy it up.

I would like to get the square edges back and fill the pitted areas before repainting. Before filling the front of the sill I would clamp two straight pieces of wood as shuttering. This would be to get back the square top/bottom edges/corners. The depth of the fill would be from zero to half inch and for some of the lager voids on the edge/corners I was thinking of firing in some stainless steel staples as a key for the filler. I would possibly use some solvent based penetrating stabilisation solution on the sandy areas before filling.

In the past a few cracks have been repaired, some with cement/sand and others with an epoxy putty and the former has proved a lot more reliable. The epoxy putty hasn't bonded too well to the interior of the sill and now can be prised out with ease (new cracks formed at the putty/sandstone boundary)

What would the group recommend as the filler material, a cement/sand mortar mix (what mix?), a cement based exterior filler, Toupret/Murex as sold by Toolsatan, a n other ?

No colour matching is required as the whole lot will be over-painted.

[Dave Liquorice]

I wouldn't bother, it's character. Recently spent half a day with and SDS chisel hacking off the cement fill on one of our stone window cills...

[stuart noble]

The new types of ready mixed filler/plaster in tubs (sold in various guises, but this one is good

are based on microspheres. IME they are excellent for exterior work and a *tiny* amount of water thins them to a more workable consistency for skimming etc. The TS Pro Fill 2 part stuff would probably be more reliable but it's very gloopy, and gets expensive. The sill would also need to be sealed with pva as the polyester won't bind to anything slightly crumbly

[newshound]

I think I would either use lime mortar or perhaps car body filler. Cement repairs are often done with strong mixes, which is why they soon look like repairs. I wouldn't try to make the sharp edge too sharp.

[harryagain]

You can get chemical based repair kits to fix this, some "plastering" skills required. There is something to stablise the powdery stone & then some "supergluey" compound to plaster on. This sortof thing.

[meow2222]

If I read correctly you say sand/cement has worked well, so what should you use. Well... :)

Lime has the advantage over cement that it doesnt damage the stone when it eventually gives up. Downside is the long setting time. People often deal w ith that by adding 5% cement, its not ideal though. Ideally lime & pozzolan , but since the stone's rather damaged, cement might not be any disaster.

NT