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Old chalky patch in plaster?

I recently bought a Victorian-era house. We had wallpaper removed in several rooms and found horsehair walls that are for the most part in remarkable shape. In several areas, the surface is a chalky white though. At first I though it was calcimine, but now I'm not sure. It seems to have been used as a patch in a couple of small (2-3 inch round areas) and in one large area. The large area butts up against a piece of drywall (I think some plaster had to be removed at some point to work on the heating ducts in the wall) and the white chalky material is covering the mesh joint tape.

Any idea what this is? Can I paint over it? In some areas it is covered with a thin layer of what looks like paint that has bubbled and peeled. We were considering covering a small area with a heavy-duty oil primer and letting it sit for a few days to see what happens.


Re: Old chalky patch in plaster?

The white chalky stuff is probably either calcimine or it could be thinned plaster of paris. In either case, I would advise you to
try to remove it. Nothing is going to stick to it very well - neither paint nor patching materials like joint compound.

A light go at it with a wire brush can remove quite a lot, although this will be dusty. Then a large sponge dampened and rinsed repeatedly can take off a lot of the remainder.

When you have removed the stuff, let it dry, then brush on some PVA primer or some kind of universal sealer (water-based). When that is dry, you can repair the bad spots with joint compound, using several coats if necessary.

This may work, but since I can't see the stuff for myself, I can
only propose what I have done before in similar situations.

Good luck.

Re: Old chalky patch in plaster?

Thank you. In places where it is thin, it is srubbing off with Dirtex and a scrubby sponge. In places where it is thick or covered in paint, a wet sponge and a metal scraper seem to be doing the trick.

thanks again

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