Priming porous wall before skimming

Question

I have ripped off the wallpaper from the walls of my bedroom, and I am now planning to fill the holes in the wall, skim the walls with plaster, and then paint. I’d like some advice on this as it’s the first time I try. On another forum, I have been advised against using PVA for priming the walls before skimming, because the walls and old plaster are porous (the bedroom is in an old victorian building in Edinburgh). Do you have any suggestion on what product I could use for priming the walls if not PVA? Or is it enough to sand and wash the walls with water before plastering? Also, any suggestion on good ready mixed plasters that I can find in UK? (or if you think that it would be better to use plaster in powder, could you explain why and possibly suggest good brands?)

Here are a couple of pictures of the walls to give you an idea of the job: Cheers :D

Answer 1

You should ideally use a plaster bonding agent between old and new plaster. These may contain PVA but don't really form a covering layer like a primer for paint - they will soak into plaster and leave a sticky surface. Many products marketed mainly as a concrete bonding agent will also be listed for use with plaster.

I have skim-coated numerous plaster walls in my house (built 1893 in the Northeast US). Here's what I did and recommend:

1. Remove wallpaper
2. Identify any cracked or loose areas and either repair or remove. To repair, assuming wood lath: drill holes through plaster but not the lath behind it, inject adhesive, and use a plaster repair washer and screw to clamp the plaster onto the lath until it's dried. To remove: chip out loose plaster and expect to replace it completely.
3. Fill any remaining holes/cracks.
4. Skim coat the entire wall.

Now some product suggestions.

For repair, I have used the "Big Wally's Plaster Magic" kit, which includes a bonding agent in a spray bottle so you can pump it into the holes you'll create in the plaster. I've also used Loctite adhesive with no bonding agent. Both have worked totally fine for 10 years. If I had to do more I'd probably just use the Loctite as it's about a third the price.

For skim coating, I've used setting-type drywall compound (Easy Sand brand). I recommend avoiding pre-mixed compounds because they are likely to shrink more and aren't quite as sticky as this "hot mud". If you're skilled enough to use actual veneer plaster (as opposed to compound that you can sand as much as needed to make a smooth surface) you should prepare the surface according to the directions of the mix you buy.