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skimming year-old plaster board |
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| message from robin.pain@tesco.net on 17 May 2004 |
A plasterer told me he once started work on a ceiling and it went off
straight away. It turned out the plasterboard had been up too long and
had dried out.
I am just about to skim a couple of small areas, the biggest is 4
square meteres and the board is bone-dry without any smell. It has
been up for a year.
Shall I just go for it, or in your experience, would you wet if first?
If so how?
Cheers
Robin Pain
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| David W.E. Roberts replied to robin.pain@tesco.net on 17 May 2004 |
An alternative to the plastere's big bristle brush waved at the wall is a
pump up garden sprayer - the kind you use for weedkiller.
Filled with clean water this can give a fine misty spray and be used to wet
down large areas quite quickly.
For a small area a hand sprayer (just a bottle with a trigger top) would no
doubt do the same.
HTH
Dave R
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| robin.pain@tesco.net replied to David W.E. Roberts on 18 May 2004 |
I think the hand sprayer is a great idea, it's only a small area.
[I built my extension with the usual cheap Celcon building blocks but
I could never get them wet enough, long enough, with the bucket of
water/paint brush method (in the summer heat). It was more exhausting
leaping up and down painting it with water than applying the render so
in the end I just blasted the wall with a hose pipe.
The original part of the house (1955) used a denser block, whitish,
with a cavity, this only needed a single touch with a wet brush and
you *had* to wait for fifteen minutes or more before you were *able*
float it.]
Robin
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| Alan James replied to robin.pain@tesco.net on 17 May 2004 |
Strange, I thought that was why it was called "dry-wall". Old plaster on
the other hand goes off quickly.
Alan
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| Lobster replied to Alan James on 18 May 2004 |
I don't follow this at all - my immediate reaction was "the plaster's
having a laugh, surely...". But enough folk have responded to make me
think it must be true. Could somebody explain?!
Firstly, plasterboard is encased in paper, so the finishing plaster
has no direct contact with the plaster component of plasterboard.
Secondly, plasterers are commonly required to apply a skim coat over
an old, knackered plastered ceiling to provide a decent finish prior
to redecorating; and you don't get plaster much older than that! So
I'm puzzled.
David
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| robin.pain@tesco.net replied to Lobster on 19 May 2004 |
Plaster board is porous. That's why building regs specify foil-backed
boards for ceilings (as a vapour barrier).
Tradesmen are quick, you may not notice the little touches they do
like e.g. damping down, and *that* is the trick.
I am just a fair-weather amateur so everything I do is in the summer.
The mistake I consistently make is too dry e.g. I have to constantly
remind myself "make it wetter than you think" (the bricks, etc, not
the mix).
If you inadvertantly saturate some bricks, no probs they will be
useable in half an hour (brick-laying-wise), but when a barrow-load of
render falls off a too-dry-wall it is most annoying and hard work
reviving it by hand not to mention a time-waste.
Robin
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| robin.pain@tesco.net replied to Alan James on 18 May 2004 |
...and when new plaster touches old plaster, at least that's what the
literature says so I have always kept everything spotless and thought
it mattered until:-
The chap who skimmed the ceiling turned up and had a good laugh at my
plaster's wheel. He produced a 450 watt electric drill with a paint
mixer. The mixer was covered in old plaster. He mixed it in a large
plastic paint container, about two feet tall and about 9 inches across
that also had old plaster stuck around the bottom.
He mixed up a huge amount so that the thing was nearly full and I
could hardly lift it. By the time I had mixed up half as much again
and staggered in with it, he had most of the first mix up.
So this "remove all traces of old plaster" is a myth? (If mix large
amounts and you're quick).
Robin
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| David W.E. Roberts replied to robin.pain@tesco.net on 19 May 2004 |
<snip>
<snip>
When a small amount of new wet plaster touches cured plaster (new or old)
then the porous and dry nature of the cured plaster sucks the moisture out
of the new plaster, which 'goes off' immediately.
I have seen this whilst filling in the gap at the top of the wall against a
brand new plastered ceiling.
Not a big problem, but if I was trying to put a thin skim on an existing
porous surface I would either damp it down very well or put on a thicker
skim :-)
I don't think that small amounts of old plaster (relative to the volume of
new plaster) in a mixing tub have a magic 'curing' effect; besides which
they get very wet as the plaster is mixed.
Having said that, my 'pet' plasterer uses a sod-off big drill (T shaped
handle) with a mixer bit in a plastic tub, and these are relatively clean.
As labourer for the day I tried to leave them at least as clean as when they
arrived :-)
Makes sense - clag on the bucket makes it heavier and clag on the mixer
maxes it less effective, and if bits break off then the plaster had hard
lumps in it which can make thin skims a bit lumpy.
The main thing I learned from watching (apart from that I'd never get that
fast) is that the mix is far runnier than I would have expected.
It didn't seem reasonable that something like a thick cream would stick to a
wall or a ceiling when it was almost running off the spot board.
However runny is good:-)
I also learned not to worry too much about the initial application - once
the plaster starts to go off it can be smoothed out remarkably well with a
float and a wet paintbrush, but trying to get a perfect finish when it is
just up and still very wet is counter-productive.
Not that the plasterer had this kind of problem - his initial finish is
better than I could do after smoothing off.
He just whacked it up, sat down and had a fag and a cuppa, then went over it
again and finished it off.
Cheers
Dave R
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| BigWallop replied to robin.pain@tesco.net on 17 May 2004 |
Wet it, but not to much that it is soaking, with a basin of water and a big
paint brush. Remember to shake the excess water off the brush before you
swing it above your head.
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| robin.pain@tesco.net replied to BigWallop on 18 May 2004 |
Thanks Big Wallop, that was hint I wanted to hear, (I found out about
the water on the head thing already)
Robin
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