Preparing friable plaster for painting

Question

We have a wall that has not been looked after at all. Rising damp caused peeling. Other places just peeled anyway. It probably wasn't repainted in at least a decade or more, maybe 2 decades.

So, we scrubbed and scraped it down with a wire brush and putty knife to remove loose and flaky paint. We will probably need to spray with a pressure sprayer too.

Not all the paint comes off or will. So we are left with areas that are still painted, areas that are bare plaster and areas that are a mix.

In addition, the plaster is mostly sand and is very friable.

What is the best option to penetrate and 'save' the plaster but to also overcoat where the paint still adheres?

I thought bonding liquid but apparently that will not penetrate well and really only bonds the top particles to each other but not to the layers below. Will plaster primer be ok over old paint like that? That should penetrate better I think especially if I thin it. Do I use water-based primer or solvent based?

Any other options?

Answer 1

I would use a thinnish mixture of thin-set FlexBond mortar, applied with a large cement finishing trowel, on all plaster surfaces. Use a large paint brush for painted areas. If plaster is VERY delicate, you can use admix instead of water to mix the thin-set. This product had a binding polymer that gives flexibility and a great mechanical bond.

After it dries, you can use thinned joint compound and a roller to apply texture. Allow complete drying, then lightly sand all surfaces with 100 grit sandpaper. If uneven surface occur, apply second coat to “low areas.” The mud mixture will adhere well to the mortar undercoat.

Once second coat of texture is dry, apply two coats of latex paint. Semi-gloss or matte.

I have used this technique on restorations of bathrooms and damaged walls in older homes.

Answer 2

The guys at Plascon explained to me that bonding liquid gets the best penetration over other priming methods. I should probably replaster but since that's not affordable here's hoping for the best!