Painting help please...

digannio

I've encountered an annoying problem with an area of paint in a house I've moved into and wondered if any of you DIYers (or pros) can come up with a solution.

Under a bedroom window sill there is an area where I can't get the emulsion paint to take. It's clear that this area has a different sort of plaster to the rest of the room, probably from when the window was replaced and it is darker. All the paint below this 4 inch layer of plaster is fine as is the rest of the room.

I sanded down and applied a mist coat and top coat but the paint started to flake again and form a dusty layer on top. It's not damp.

Any solutions or products to solve this pesky problem once and for all?

Bendy_House

Are you sure it isn't damp?

When you sanded it, did you go as far as to 'key' - slightly roughen - the plaster surface with, say, 120 or 180 grit? What colour is this plaster?

Anyhoo, there are a few products which should prime and seal this plaster ready for a top coat (assuming it isn't damp...). One is Zinsser BIN (not sure which actual product - there's a few, but one is a 'stabiliser'*), and the other is something like Everbuild 406 'Stabilising solution'.

I suspect 'B-I-N' : https://www.zinsseruk.com/product/

I'd probably go 'Zinsser'.

neilmcl

It may not be damp to the touch but it sounds like it's damp underneath causing efflorescence to come through.

digannio

Thanks for the swift responses. There is a distinct powdery coating on this section if that suggests water getting in. There is no problem with the plaster lower than this 3 to 4 inch section below the sill and right down to the skirting is fine. The house has a DPC and was also given a chemical damp treatment 20 years ago and still under guarantee against any rising damp (which i guess this woukdnt be). . The surveyor's report said no evidence of damp in property. Are there some steps I could try in case water getting in. The sealant below sill on outside looks OK.  Here's some photos externally where the issue is. I couldnt see any obvious gaps to let water in from the outside although water does drip down from the plastic verge caps above there when it rains. And its clearly not rising up from foundations as no evidence at all 

stuart45

The window looks like it wasn't part of the original build, as the outer reveals look like an angle grinder has been used to cut out an opening. The fake stone(concrete blocks) have been used as an infill. 
The trouble with jobs like this is that you don't know if they used any insulation to match the existing walls, or how they did the details under the cills. 
It could be that you have a cold spot in that area attracting condensation. 
On a property of that age with a DPC you shouldn't need any chemical damp proofing. 

plumb1_2

If no damp use a stabilising solution, sand the paint off first before applying 

Bendy_House

I thought you need to be able to fully discount it being damp before you go any further.

If it is damp, then it's unlikely to be from underneath the outer cill - that's pretty well protected. I think it more likely be from the window itself, or its sides. Any water getting in around the frame in any of these areas should then be dispelled out via drain holes above the cill, but if this hadn't been done properly, there's always the chance it gets inside instead.

Don't know what to suggest - other than some method of determining the presence of damp. Perhaps aim a hose all over the window?!

twopenny

I'm not entirely sure you should discount water from under the sill.
I get a damp wall under a patio door sill. There was a post on here not long ago with the same problem. 
Usually windows are more protected by the gutter overhang. Would be worth getting your wetsuit on and standing in the rain to watch wwhat  happens if only to discount one option.

digannio

Thanks for all your input. It has helped me work out an action plan. I'm getting the dry verge caps that are dripping rainwater onto the cill replaced, then I'll fill any small gaps I can find around the sides of the window and the cill, then I'll sand right down to get a proper key, followed by stabiliser to prime and seal for the emulsion. Hopefully job done and you'll hear no more from me