Plastering woes

[Graham Jones]

Last weekend I attempted my first skim. It was onto a masonry wall already bonded with what I assumed was undercoat plaster.

All was going well until I noticed the wall was sucking all the water out of the plaster and small cracks were appearing. The plaster was unworkable within minutes of applying. I managed to rescue it by throwing lots of cups of water at it. The wife was horrified!

The same thing happened with the second coat. After polishing this the best I could, I applied a third coat which went on OK and I managed to get quite a good finish.

Should have sealed with PVA first I hear you all shout. The thing is I did!

I have another wall to do this weekend and have noticed what I thought was undercoat plaster looks actually like sand/cement with lots of tiny stones in it. Would this suck water like a camel? My plan is to apply two coats of nearly neat PVA.

Does this sound reasonable?

Anybody know what went wrong?

Thanks,

Graham

[Lobster]

I wouldn't use neat PVA, you don't want to create a hard membrane which might even peel or flake off; use it dilute as normally recommended (I forget the dilution, 1:4 or 1:5 I think?) Should be fine. Whether the substrate is affecting things though, I dunno.

One no-brainer to rule out though... is your plaster old by any chance? Once te bag is opened it deteriorates very quickly, but imperceptibly other than once mixed, it sets *incredibly* rapidly!

David

[Andrew Gabriel]

Cracking can also be caused by the coat being too thick. Finish coat plaster shrinks a lot as it goes off. For a thin coat, this happens by the coat getting thinner which you don't notice. For a thick coat, it cracks through also. Should be 1-2mm per coat, and normally 2 coats (see a posting I did recently explaining the purpose of the two coats).

For a very absorbent background, you need two coats -- a 1:5 dilution with water to soak in and act as a binder, and then a 1:3 which won't soak in as fast, and you plaster onto it whilst it's still slightly tacky.

No, you need to dilute the PVA as above.

[Mark S.]

You don't mention any time scale to the application of the coats but mention polishing.

At a guess I'd try making it a more "sloppy" mix next time?

Mark S.

[Peter Parry]

How old was your bag of plaster?

[Mike]

It may be the undercoat is something like Tarmac's Lime-Lite product which drinks water. Sponge it down before plastering over and use a sloppy mix.

[Graham Jones]

Thanks for all the advice. BTW the bag of plaster was bought from B&Q the day before.

[Andrew Gabriel]

It should have a 'best before' date printed down the side, usually with a giant dot matrix printer. Sometimes the writing is even legible. :-) However, I don't think that's what was wrong in your case based on what you said. Old plaster just tends to set faster, which actually makes it very useful for small patch/repair jobs. Since you were able to solve the problem by throwing water at it, this implies it dried out, but hadn't set.

[Mike]

Probably out of date then :-)

Look for the best before sticker. I've had several near their date.

[Graham Jones]

August 2005 so I guess no worries there then.