Re-skimming old plaster repair: first with sand+cement or just finishing coat

[jtravs]

Hi all,

While attempting to strip wallpaper in my 1912 built Edinburgh flat, chunks of the plaster started coming away. After finishing stripping I thought I'd clean up the missing plaster and attempt to patch it, but then whole slices of the plaster were coming off. So I pulled it all down. It mostly just peeled off by hand.

I now have some kind of backing plaster/render. The outer plaster that I removed was about 5 mm thick and very smooth. What is left is rough and quite sandy/crumbly. I'm guessing it is sand+cement but really have no idea. I'd rather not remove this too as I want to keep things simple. It seems more or less sound.

When chipping off some of the harder parts of the old plaster top coat I made some holes/dents in what is beneath, and there are a few bigger missing pieces. So the question is what should I do?

First, I don't want to get a pro in. This is my first house and I want to take the liberty of cocking it up myself first (I have no experience in anything related to DIY).

My first plan was to just follow the copious guides to skimming and do the following: cover what I have with 1:5 PVA, let dry, coat again with 1:3 PVA, followed when nearly dry with a layer of multi-finish plaster, using that to flatten out the wall before a second finishing coat.

But then I read that the finish plaster should only be about 3 mm thick, which is mostly fine, but in some areas I'll need to fill in indentations at least twice this if not more.

So now I'm thinking of putting up a thin layer of sand+cement+(possibly lime as I understand this helps in these older buildings), and floating that to get a nice flat wall before the plaster skimming. Alternatively I was thinking of using bonding plaster, but I've read that doesn't work as well in these old buildings and that an amateur like me might have more luck with the more patient sand+cement mix.

What do you think?

 

[JohnD]

photos would help.

What colour is it?

lime is usually cream/grey/white; gypsum is usually brown or pink.

Some may have sand in it, or chopped animal hair.

in some districts lime was used, with a hard plaster skim, and these districts may still use sand and cement.

If yours is an old house you would do best to match the original materials, and if it is on stone, you need someone with local experience.

If you are a beginner, it is hard to plaster a wall (more than just patches and chases) and sand and cement is I find easier. I am a DIYer and have never done lime, though I have had a specialist remake old L&P lime ceilings. They were very thick, and soundproof.

 

[jtravs]

Here is a picture. Most of what you can see is the backing. At the bottom by the ladder you can see a little bit of the old finishing plaster that I haven't removed yet.

 

[JohnD]

to my eye it looks like gypsum plaster, by the colour and the trowling marks. Did it fall off when you used a steamer and overheated it?

A close-up, in your hand to show scale, will help.

 

[jtravs]

I didn't steam it, just sponge with cold water and scraper - and I was quite gentle. Parts of the plaster were off the wall even under untouched wallpaper. i.e. you could push the wallpaper and see/feel the plaster beneath move a bit. Once stripped those bits came off immediately, but gently pushing the scraper on the edge of what remained caused huge junks to easily peel off.

It may be that it is much more recent, I believe teh house was renovated about 10/15 years ago.

If it is gypsum, then I guess I could just use bonding to fill in the deeper holes and then finish plaster? Or do you think I could do it all with multi-finish?

This picture is slightly closer from before I peeled it back. For scale I think (from memory) that the lines/grooves are about an inch apart.

 

[JohnD]

you can also recognise gypsum by the smell when wet, I think it is. The lines in the backing coat are scratched in with a "devil" (think of a scrubbing brush with a row of nails instead of bristles)

I think you will find it easier to use a backing coat and a thin skim of finish, or it will crack. It is easier to get a thin skim flat.

You may find a pro plasterer comes on here and knows better, especially if local, perhaps at the weekend.