Sandstone Repairs

Hi

Not sure which forum was better for this question, building or decorating.. apologies if this is the wrong choice

Basically am restoring sandstone window sills/surrounds.

They seem to have been overpainted several times in the past, the paint now is blistering and there are also large areas which sound hollow, so i think some sort of filler has been used.

I am stripping the paint off, the loose stuff at least, and leaving the paint which doesn't seem to want to come off. The loose filler is also being removed. Essentially taking it back to stone as much as possible. Is this approach sensible, or is it better just to overpaint whats there already? Am using a heatgun and scraper.

Do I need to re-seal the sandstone before painting it (am guessing so?) and also is it OK to "seal over" the paint which I cannot remove?

Within the gaps between the blocks (I assume they do not just rest atop of each other?!) some of the filler material was wet. It looked like plaster or car body filler. Doesn't seem right? See second pic, and also green mould type stuff.

Thanks
Mike

Gaps should real be pointed. It will be difficult to match your stone colour tone exactly, often dries lighter and you depend on weathering to discolour overtime.
When repair natural stone, I try to source a stone type that is similar colour/tone to the one being repaired, grind that down to dusty particles to use as aggregate, then blend the colour using white and ordinary portland cement.
The are normally require priming with a suitable external PVC prior to repair.
The repair mix will be trial and error until you have the correct ratio of aggregates to cement that give you the most suitable dried colour.
So you will have to do some sample design mixes prior to application.

Thanks. What you say makes sense.

I have used a white filler to fill those gaps as I am going to paint the whole lot in a different colour to what is there at present.

Using an external pva on bare stone I advised , if I understand correctly ?

Hope I haven't screwed up by using masonry filler. It has set fairly hard.

Thanks

Are you sure that your "sandstone" is not actually "Ham stone" a synthetic stone often used in Victorian houses? It has a more even texture without any grain.
Frank

Hadn't considered that..! Could be. How could I tell and what would the implications for the way I am doing this? Thanks

Edit...hamstone is not synthetic http://en.m.wikipedia.org/wiki/Hamstone

Years ago I read an article on the origins of Portland cement. it had a lot of other nuggets of info in it and Ham stone was definitely mentioned, must be a synthetic copy was being discussed.
You could try here :- www.periodproperty.co.uk/forum
Would your location give it away?
Frank

Think I'm just going to over paint the lot.. strip back loose stuff, fill the holes, paint over the cracks. Pretty much what I did on the inside haha. Not painted exterior before hence wasn't sure what to do.