plastering

[Stephen]

Hello,

I have been stripping some wallpaper and some plaster has come of from behind it. I did use a steamer, is this to blame? I would have thought that a steamer would cause air bubbles trapped under the plaster (between layers?) to expand and cause the plaster to crumble, so is it a sign of the plaster being bad, rather than the steamer's fault? How could a steamer damage good plaster?

I don't think I held the steamer too long in one place as three walls escaped without injury but the fourth wall had a few areas where the plaster fell off. It's only the top layer of plaster that fell off, leaving a very sandy render underneath. I tapped it to see what was hollow and picked off the loose bits by hand.

I used to buy Wickes ready mixed tubs for about £7 but they are now nearer £12! I saw that a small bag of plaster powder was only a few pounds so decided to try and mix my own.

The instructions were a bit vague so I wasn't sure quite what consistency I wanted. I thought if it was too runny it might take forever to dry and might slide off the wall. I thought if it was too dry it might be lumpy and crack.

I mixed some that seemed to apply ok but overnight it has cracked. I understand this means it dried too fast. (I did not have any heating in the room: the rad was removed for decorating).

Does this mean I need to make the mix a bit wetter next time?

How do I treat the cracks? Ca I just fill them with more plaster/filler?

Thanks, Stephen.

[Tim S]

Stephen coughed up some electrons that declared:

You could damp the wall down first - my dad used to do this and his plaster patches didn't have any problems. Either flick a wet brush at it or use a plant sprayer.

My dad undercut them into an inverse V shape with a stanley knife. That worked, but was a bit of a faff. For cracks, I've brushed PVA on, then used a bit of fine filler with good results. PVA is optional - but I was trying to stop the crack edges moving and cracking again (near a doorframe on plasterboard).

Cheers

Tim

[Andrew Gabriel]

Drying and setting are two different things. The plaster must stay wet until it has set, or it will stop setting -- setting is a chemical reaction with the water. Once it's set, it can then dry out, and it doesn't matter how long that takes. Finish coat plaster is mixed up quite sloppy -- more sloppy than you might imagine if you've not done it before.

You haven't said what plaster you used and how you applied it, so it's difficult to help. Plaster shrinks as it sets (finish coat particularly), so it will crack if applied too thickly.

2mm is probably about the safe limit for one finish coat.

Plaster doesn't make a good surface filler. Use something like pollyfilla.

[Guy Dawson]

The sandy render is probably the reason why the plaster came off.

Did you use anything like dilute PVA to seal the sandy render?

Guy

-- -------------------------------------------------------------------- Guy Dawson I.T. Manager Crossflight Ltd [e-mail]

[Phil L]

Does the plaster resemble a crocodile's back? - or similar to the markings on a tortoise shell? - with plaster, it's not a case of getting it on the wall that's the problem, it's the problem of keeping it wet until it's set - it can't set dry, so if you rub your hands over the plastered area, do pieces of it come away? - is it leaving a lot of powder on your hands?

1) the basecoat plaster needs wetting, then wetting again and finally wetting again, but this third time with PVA solution. 2) once all the plaster is on the wall, this is when the work begins, not stops. 3) if it cracks up after you've finished, it will probably last as long as a snowman in August, and no, you can't fill the cracks with anything, unless you want to make matters worse - see #1

[Stephen]

Thanks, yes I had done this (I once saw pro do it when he plastered a wall for me).

[Stephen]

In that case, mine was probably a bit too dry.

Probably applied too thick too ;)

I have used a surface filler to fill the cracks, thanks.

Forgot to say it was Wickes multipurpose. BTW what stops their premixed tubs from shrinking and cracking?

[Tim S]

Stephen coughed up some electrons that declared:

There is an exceedingly good video somewhere here I came across yesterday:

the last video here:

bloke shows how to do a whole wall rather than patching, but you get to see the consistency of the mix quite well.

Cheers

Tim

[Stuart Noble]

The idea of wetting surfaces has really been outmoded by pva I think. It's certainly a more reliable system for d-i-yers. I've known surfaces that would absorb a bucket of water and still give you cracked plaster. Pva never fails IME.

[Stephen]

Thanks. Please can you describe your method. Do you just brush a dilute mix of water-pva over the surface and allow it to dry first? How dilute do you mix it? I shall switch to your method in the future. Thanks.

[Andrew Gabriel]

Another issue is that many of the thermal blocks used nowadays don't like being wetted much -- they expand and contract enough when wetted and dried to crack themselves and lose key with the plaster.