Fixing plaster damage that has been painted over

Question

I recently bough a new house and it came with a fair bit of plaster damage in the walls. The biggest problem area is this patch of water damage where the gutter leaked down the exterior wall. This damage appears in two rooms, one of which has been painted since the damage happened.

I've never done a plaster patch before. I think I understand the process and have the tools I need. I should be able to fix the damage in the unpainted room, but I'm wondering if I have to scrape away or strip the paint before I patch the painted room? If I do, does anyone know the best way to go about doing this? Is this even worth worrying about?Thanks so much.

The unpainted damage (in the room above):

The painted damage (in the room below):

Answer 1

A paint job is only as good as the prep work that comes before it.

• If anything is loose, scrape it off.
• Sand a nice smooth transition from the existing finish plaster layer down to the layer below it.
  • Note that this will likely leave a visible line unless you really feather the plaster back, and it will take quite a bit of sanding to make this smooth enough to not print through the final paint.
• A better option would be to fill the depressions in the plaster and make a nice smooth surface.
• If the existing paint is firmly attached to the wall, there's no reason to remove it.
  • If the paint is a high gloss, it might need a light sanding to allow the next coat to physically bond to it well.
  • Often, a ScotchBrite™ type pad will do just fine for scuffing existing paint. It doesn't need to be thoroughly sanded, just enough to give it a good "tooth".

Answer 2

Assuming the walls are completely dried out...

You should remove the damaged plaster back to give a smooth surface & re-plaster the new surface. Then paint with a water-based paint.