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

Graham Jones wrote:

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.

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

In article ,
Graham Jones writes:
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!

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).

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!

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.

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?

No, you need to dilute the PVA as above.

--
Andrew Gabriel

On Tue, 19 Apr 2005 19:25:07 +0100, Graham Jones
wrote:

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


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.

On Tue, 19 Apr 2005 19:25:07 +0100, Graham Jones
wrote:

Anybody know what went wrong?

How old was your bag of plaster?

--
Peter Parry.
http://www.wpp.ltd.uk/

"Graham Jones" wrote in message
...
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.

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 wrote:
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


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

In article ,
Graham Jones writes:

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

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.

--
Andrew Gabriel

"Graham Jones" wrote in message
...

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

Probably out of date then :-)

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

Andrew Gabriel wrote:
In article ,
Graham Jones writes:

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


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.


August 2005 so I guess no worries there then.