plastering

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Stephen

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Jan 12, 2009, 9:04:10 AM1/12/09
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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

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Jan 12, 2009, 9:08:20 AM1/12/09
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Stephen coughed up some electrons that declared:


> 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?

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.



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

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

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Jan 12, 2009, 10:16:43 AM1/12/09
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In article <r1jmm45sn6q2b2j02...@4ax.com>,

Stephen <inv...@invalid.org> writes:
> 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.

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.

> 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?

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.

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

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

--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]

Guy Dawson

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Jan 12, 2009, 12:04:33 PM1/12/09
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Stephen wrote:
> Hello,

> 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.

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
gn...@crossflight.co.uk

Phil L

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Jan 12, 2009, 3:14:09 PM1/12/09
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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


--
Phil L
RSRL Tipster Of The Year 2008


Stephen

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Jan 13, 2009, 5:28:36 AM1/13/09
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On Mon, 12 Jan 2009 14:08:20 +0000, Tim S <t...@dionic.net> wrote:

>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.

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

Stephen

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Jan 13, 2009, 5:30:27 AM1/13/09
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On 12 Jan 2009 15:16:43 GMT, and...@cucumber.demon.co.uk (Andrew
Gabriel) wrote:

>Finish coat plaster is mixed up quite sloppy -- more sloppy
>than you might imagine if you've not done it before.

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

>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.

Probably applied too thick too ;)

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

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

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Jan 13, 2009, 5:50:15 AM1/13/09
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Stephen coughed up some electrons that declared:

> On Mon, 12 Jan 2009 14:08:20 +0000, Tim S <t...@dionic.net> wrote:

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

http://www.ultimatehandyman.co.uk/PLASTERING_CENTRE.htm

specifically the last video here:

http://www.ultimatehandyman.co.uk/PLASTER_SKIMMING.htm

The 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

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Jan 13, 2009, 7:05:27 AM1/13/09
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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

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Jan 15, 2009, 5:15:10 AM1/15/09
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On Tue, 13 Jan 2009 12:05:27 GMT, Stuart Noble
<stuart...@ntlworld.com> wrote:

>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.

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

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Jan 16, 2009, 6:09:38 AM1/16/09
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In article <bs%al.19492$Sp5....@text.news.virginmedia.com>,

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.

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