Re skim old ceiling

Hi.

I can't tell from your photos what that residue is either. But, as long as the surface is intact, firm and not flaking, you should be able to treat it with a couple of coats of PVA (or use 'proper' pre-skimming stuff) and then skim.

What I tend to do in these situations is to first coat with slightly diluted PVA - say 10-15% water - and allow to dry. The I apply a neat coat of PVA and also allow that to dry. I have found the resulting surface ideal for skimming over.

I did once try the 'recommended' way and that was to allow the second coat to simply go 'tacky' and not fully dry, and it was like trying to skim on oil - I applied (or tried to) the first trowel of thistle and then spent a bemused 10 minutes pushing it all over the wall like a game of air-football. I chust couldn't get the darned stuff to spread.

If the PVA is allowed to fully dry, its surface will still 'reactivate' with the application of the skim coat and will bond to it nicely.

Alternatively you can use proper stuff like 'Blue Grit' - as its name implies it has a gritty texture which is meant to help the bonding process.

Anyhoo, if you apply PVA and it sticks and dries and doesn't lift the surface or nuffink, you can be pretty confident that it'll take your skim coat nicely.

 

I think everyone has their own recipe for this....! but yes I agree leave as is. I would go over with 6:1 and allow to dry and then 4:1 and skim whilst damp. The theory is that plaster will adhere better if the pva is damp but lost count of how many times i have tried to remove dried plaster from variety of unprepared surfaces I wouldn't worry about it too much. You will need 2 coats of plaster however as the first will go off quickly! The second immediately after the first will trowel on much thinner and smoother, its common to go over a ceiling 4-5 times depending on skill and how quickly it goes off.
So two coats of plaster then immediately trowel over from the beginning to take out high spots and smooth, then back to beginning for final trowel and the a final with water to polish. May be omitted if you are painting!!