Skimming over emulsion on brickwork

[Kevin Brady]

I have a downstairs loo in a side building attached to the house, which I'm doing up.

Internal wall is brickwork with a light coat of white emulsion over (very light coat in places, you can see brick through the brush strokes). The ceiling is a in-situ concrete, with tiny ridges where the forming timbers were removed. The ceiling is similarly treated to a watery emuslion finish.

Ideally I would like to give both surfaces a skimcoat of (bonding?) plaster. Do I first need to remove the watery paint from the bricks/concrete? Is bonding plaster the best for skimming (2 coats max)?

Many thanks.

[ben]

On brickwork its ok and probably wont need a pva bond.

[John Schmitt]

[wants to skim semi-external loo]

The paint must go. Guessing at the age of the property, it is probable that under the emulsion is distemper, a composition of powdered chalk and animal glue. Even if emulsion is the only paint, it is still likely to compromise the bond. Use an angle grinder with a cup-shaped wire brush attachment, eye protection, essential (they tend to shed wires), to remove the paint. The ceiling will need a once-over too as there may be release oil on the surface. Give it all a coat of PVA to promote adhesion and reduce suction (Giving you longer to work the plaster).

Probably it will be cheaper to buy a cheap angle grinder than to hire. While the cheapo tools are not as well made, for the average houseowner, they serve the purpose.

Bonding plaster is normally the substrate, finish plaster is the final layer to provide a smooth surface.

John Schmitt

[ben]

The paint won't have to go, if its as he says (the brick is showing through the brush strokes), then the plaster will penetrate through this and aquire a firm bond to the brickwork as it drying out. Bonding plaster is the correct technique first and the a skim of finishing plaster, although if he does a smooth job with the bonding plaster there will be no need for finishing. I recommend he use dry wall plaster for the bonding.

[John Schmitt]

Can you explain the mechanism behind your theory? Do you understand exactly how plaster adheres to its substrate? Are you, or have you ever been, an adhesives chemist? A basic test for paint adhesion is to crosshatch it with a sharp blade at 2mm spacing over 20mm square, firmly press down sellotape and then after at least a minute (elasto-plasticity comes into play)pull it off sharply. If a single flake of paint comes off it should not be trusted as an adherend.

I was thinking of fixing plasterboard, but the toilet in question may well have been designed on the small side, and so the resulting loss of dimension may be unacceptable.

John Schmitt

[Andrew Gabriel]

Firstly, are the walls or roof damp as is likely if it's just a single skin wall? If so, you are going to require some special measures as the damp will wreck gypsom plaster. I'll assume not.

Unless the paint is flaking off or washes off easily with water, you shouldn't need to remove it. I would seal the surface by painting it with a 1:5 PVA:water solution though.

Then you need to apply a scratch (base) coat plaster. Bonding coat is good for this -- as the name implies, it sticks to things very well. The scratch coat should give you a flat vertical surface, but doesn't polish up and should be left rough for good keying of the finish (top) coat. Use Multifinish plaster for the finish coat. You normally do two coats of finish coat, ideally applied when the coat underneath has set (or almost set), but is still wet. If you take a break from the job and the coat underneath has dried out, prime it with two coats of dilute PVA, first at 1:5 and allowed to dry out, and then second at 1:3 and allowed to dry to being tacky.

[ben]

Are you a qualified plasterer?

I am not, but I have been plastering for the past 8 years and have gained a lot of knowledge and advice from good plasterers. Don't take this the wrong way but I think you underestimate the powerful suction&adhesion of todays plasters. If I thought for one minute that the OP would not get a sound fix to the walls then I would have gone the way you described. Theres a difference between emulsion on brick and plaster.

[Kevin Brady]

Thanks to all who responded. As is usually the case, my very brief description left a lot of room for 'interpretation'.

To clarify, the loo has one long external wall 9" brickwork, which I am dry lining with sw battens, Kinspan and 12.5mm plasterboard and skim.

The room is very narrow, so on the long internal partition (4") brick, I hoped I could get away with just a skim finish on the (wishy washy painted) brickwork.

The low ceiling, as I mentioned, was cast in concrete, spanning between the external wall and internal brick partition. There is no evidence of damp in either the ceiling or the internal partition. Due to height, again I plan to simply skim this, and use the patterns from the formwork to help as a key.

The property was built by the council in 1948, and the thin paint wash was put on in the last five years to quickly brighten the loo up before we bought the house. If I stripped the brickwork of paint then I would consider hanging my minimal handrinse basin against this wall without any plaster finish, but I dont think the brickwork is nice enough (very smooth, flush joints, little 'character').

This is the first time I'll have tried internal plastering, although I did a succesful job of a white renderer retaining wall in the garden (using a wet sponge to smooth off afterwards). Since it's the downstairs bog, and small area, I thought it would be a good place to learn my mistakes.

Thanks again for the useful comments, as always.

Regards,

[John Schmitt]

No, but I have seen the results of self-made plasterers. Very clearly some of them failed to read the manual. Unfortunately, the building trade tends to attract flibbertigibbets and ne'er-do-wells.

The fact remains that if the paint is not well bonded to the plaster, it will act as a release coating. Depending on the paint, it might even be water soluble, with predictable results. Relying on the (poor in building adhesive terms) adhesion of paint is an excellent way to produce a job which will fail.

The differences between emulsion paints are rather greater. As this paint job appears to be a makeover, it is reasonable to assume that an extremely cheap, low quality product was used.

John Schmitt