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I need to repair peeling flaking exterior paint on my stone window sill, so looking to find a suitable matching paint but I'm puzzled on what this stuff is.

Looking at the back of the paint, it looks like what feels like some sort of substrate is attached... feels like construction paper but if you look closely it is actually made of sand-like masonry. Does that mean there is some sort of masonry primer I should use when I'm painting this sill?

Or should I just scrape all the old paint and substrate off and just paint with a regular exterior paint?

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What's the best way to remove this kind of stuff, I can scrape the flat surface with a putty knife to remove, but the vertical side has an irregular rough stone pattern - should I just sand it off there?

Wide-angle shot of the outdoor sills when they were just renovated in 2015

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    A wider picture will help provide more context, those are a bit too close up. (The opposite issue of what most provide!) That said, I'd guess that the original surface was parged with a layer of concrete/plaster/stucco type material then painted and it's mostly the parge coat bonding that's failing. – FreeMan Apr 22 at 11:55
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    have it checked for asbestos – jsotola Apr 22 at 19:11
  • Is the surface tar like when it gets hot? It could be an asphalt sealer over a skim coat. – Ed Beal Apr 22 at 20:25
  • The work was done in 2015 so it's only 6 years old... @FreeMan I've upload a wide angle shot – MonkeyBonkey Apr 22 at 20:37
  • TBH, the "feels like cardboard" is very confusing, but I'm still of the opinion that it's some sort of parge coat that was then painted. – FreeMan Apr 23 at 11:20
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I believe what you think of as paint is actually a skim coat.

Skim coats used for covering stone and concrete may have the paint or a pigment mixed in.

Your photo is not clear to me and I originally did not open the question because I don’t know of a paint that is like cardboard.

I do know (cement & sand) or other stucco like coatings have been used to cover stone and concrete and make a “new” looking finish.

In some cases when the skim coat is getting weathered folks painted them and Sears used to have an “elastic” or rubber like coating that worked on “everything” if you were to believe the salesman but this was +30? Years back possibly put on top of a skim coat it might feel that way today I don’t know any one that purchased that product but believe the “sand” base is what is left of a skim coat.

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  • Is a parge coat the same as a skim coat? Would it be reasonable to just scrape it all off and just use an exterior paint that matches? Should I use a primer then? – MonkeyBonkey Apr 23 at 11:36
  • Skim coat and Parge coat are basically the same thing maybe more add mix in the parge coat it is a covering to hide the original surface. comprised mostly of sand like any mortar, skim, stucco. It sounds like not enough add mix for an exterior surface, the addmix adds water resistance and strength. The new coating was probably put on top but because the coating was failing it is now flaking or that would be my guess with the photos. – Ed Beal Apr 23 at 13:19
  • should I try to replicate the parge coat when I fix, or should I just use a brush on primer of some sort? – MonkeyBonkey Apr 23 at 18:24
  • That is really up to you. I have mixed sand with paint on a couple of projects to make a OSB wall match more closely the stucco, I doubt it lasted more than 5 years. A pro could tell the difference but the owners were ecstatic as that fix was about 10% of doing it right. Any loose material needs to be removed or if not saturated with paint it will quickly flake off a roller back and forth helps but the sills are not very wide. – Ed Beal Apr 23 at 18:44

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