skimming versus patching/filling and stubborn cracks

[DH05]

Four related questions:

1) At what point do you decide it's not worth filling and patching and it's better to skim the whole wall?

2) I've seen an "easifill" product recommended in other replies for patching because it's easy to sand. I bought some to try it out but see that it's labelled for plasterboard joints. Will it adhere over painted surfaces?

3) What do you do to assure a skim coat of plaster adheres? I had a kitchen skimmed after I removed tiles from several walls and the skim is already separating where it was applied over existing painted wall surface.

4) I'm working in a nine storey 1930s block of flats with reinforced concrete construction and all the interior breeze block walls have full length cracks that seem to move very very slightly over time. Is there any solution other than hiding under lining paper?

 

[Nestor_Kelebay]

Perhaps the solutions I'm offering won't be good enough if you're a contractor and the customer is wanting a permanent fix to the cracks.

(I don't think there's any way of permanently fixing those cracks if they're due to movement of the building.)

What I do with small hairline cracks is simply fill them with joint compound and scrape any excess joint compound off the surrounding painted surfaces. Once that joint compound dries, I then wipe down the joint with a damp sponge, thereby removing any residual evidence of joint compound on either side of the crack, but leaving the crack essentially full of joint compound. Then prime and paint over that.

You can ensure that a skim coat of plaster sticks well to a wall by first painting that wall with a solution of white wood glue diluted with water to a paintable consistancy, or with a concrete bonding agent (which will already be thinner, and will cure to for a waterproof bond). The moisture from the skim coat of plaster you put over that glue will reactivate the glue and help ensure that the plaster sticks well to the wall. I paint with diluted white wood glue all the time in my plastering work. I also will mix some white wood glue into the plaster I use to make it both stick better and dry harder when that's what's needed.

Is there any idea as to what is causing the movement in the building?

 

[DH05]

Hi Nestor, I've been filling cracks more or less as you describe, but within a few months, they reappear. I guess with a concrete building of this size there will be some expansion and contraction with the weather and things simply have to move. Having lived in California most of my life, I'd only ever worked on "drywall" walls and now I see how much better they are than than plaster over brick.

I'm in the UK now and am working through a major redecoration of the flat I live in and I'm learning that plaster is really vulnerable and deteriorates quickly. This building is 70 years old and the walls under the layer or two of paper are very inconsistent. Around the windows, in particular, there are sections a couple of feet long that have completely separated from the underlying bricks. If I attempt to open up the cracks a little, entire sheets fall cleanly off. I'm wondering whether it wouldn't be better to stop patching and just hack it all off and replaster.

As for the PVA (wood glue) under the skim coat, yes, that's what I used, but it didn't stick. I guess you wouldn't really expect wood glue to adhere all that well to oil based paint (which is what was on the kitchen wall) - although I did a pretty thorough scrub with an electric sander and coarse paper before skimming.