In article ,
Stephen writes:
On Tue, 29 Sep 2009 21:20:28 +0000 (UTC),
(Andrew Gabriel) wrote:
Browning is a particular type of scratch (base) coat.
Thanks. I had heard the name before and thought it was a synonym for
base coat; I had not realised it was a particular type of base coat.
[PVAs]
are all the same except exterior PVA (which should more
properly be called EVA, but then builders wouldn't know what
it was).
They'll all work, but EVA is a waste (it's more expensive and
won't make any difference in this application).
I once bought a litre of exterior PVA, perhaps I wasted some money
because I've only used it for interior gluing. I guess at the time I
bought it thinking it would cover all circumstances and I wouldn't
need to buy a second bottle if I ever needed to glue anything
outdoors. However for plastering, I would get through the bottle too
quickly and I agree it would be a waste.
EVA is more waterproof than PVA, but only in one circumstance,
when it's mixed with cement in mortar. When just used as a glue,
it has pretty much the same properties as PVA.
The other thing to check is if the PVA is already diluted, and
it sometimes is in the 5l containers. Plasters often buy it this
way as it saves diluting it (it's not the easiest stuff to mix
with water,
I've had a look on Wickes' web site but it doesn't go into detail
about the different types of PVA they sell. They have "waterproof" PVA
which I guess means exterior PVA and "building adhesive" PVA which I
guess is interior PVA? All of them are described as being for sealing
and priming and being "ready mixed". I am not sure whether ready mixed
means ready diluted or whether it means it is a paste not a powder?
I suspect that means already diluted.
Anyway, my plasterer used the £9 rather than the £19 bottle, so I
think you must be right. It probably is diluted which is why it is
half the price of the others, which presumably are not diluted.
http://www.wickes.co.uk/PVA-Adhesive/invt/240701
A dilute coat such as 5:1 acts to stablise the surface if it's
dusty and to reduce the tendancy of the base to suck all the
water out of the new plaster before it sets.
This is what I need in my application. I will buy the 5L one from TS
as it's a bit cheaper than Wickes. Sadly TS and BondLoc's web sites
say very little about he product. I'll have to read the label when it
arrives.
Secondly, finish coat plaster does not have any bonding in it,
and only a limited ability to stick to things. Ideally, you put
it on when the coat underneath is not quite set, and then it
will bond well with that. If the coat underneath is well set,
then it won't bond well, and if it's something smooth such as
plastic, it will hardly bond at all. To get around this, you
put a more concentrated PVA coat on (3:1 is typical)
This is what he needed to glue the plaster to the ceiling. That
explains the difference, thanks.
[about conduit]
It glues the plaster to the plastic.
I have been experimenting with chases. I saw Wickes sold bags of
mortar advertised for filling chases and TLC sell quick setting
cement. Would PVA be needed for either of these?
These are custom products, so you'll need to read the packet.
Bog standard mortar would, but a special product may include a
bonding agent in the mix.
I have used plaster in the past but it always cracked because the
chase was too thick, which meant a second coat was required on top. I
found that the cement cracked too. However, when using mortar there
were no cracks so it made a simple one-application process. I should
add that this is on a wall that will be papered over. I guess if you
have just a plastered wall you would need to skim over the mortar to
give the right finish.
Plaster shrinks a lot as it sets. Mortar much less so (and less
still the drier the mix used).
Ideally, use bonding coat plaster in the chase up to about 2mm
below the finished surface. You still need to kill the suction
with dilute PVA on the exposed plaster edges before using. It
will crack, and it doesn't matter (even makes for a better key for
the finish coat). When almost set, apply finish coat to level off
the surface. That won't crack, providing it's a thin layer.
However, buying and carrying two large bags of plaster just for
filling chases is rather silly unless you're doing loads of it.
What I actually do is keep hold of the end of a bag of outdated
finish coat plaster. Being out of date means it sets really quickly,
which is usually a pain, but in this case is an advantage. First,
I use it to make up some bonding coat by mixing it up with a _little_
PVA in it, and fill chase to 2mm below surface. Hopefully, it will
set in about 15 minutes (being old). It will crack, but again this
doesn't matter. Whilst it's setting, I can be mixing up some finish
coat (no PVA) to level off the chase. All done in 20 minutes.
Being a novice, mortar is easier to sand off if too much is put on
that the cement and the cement is stickier and not as nice to apply.
Eh? You don't use pure cement.
These worked for me as a complete beginner, but for people who can
plaster, plaster is probably the way to go.
--
Andrew Gabriel
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