Plaster and PVA

I know it is a good idea to prepare old brickwork with PVA before plastering.

I don't want to buy a great 5 ltr of it - can I use some watered down PVA Woodworking adhesive - or similar Grips Like ???? stuff.

Reply to
John
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Any PVA wood working glue will do but a 5ltr bottle of it is less than a tenner at Wickes.

Reply to
adder1969

Sure. I got 500 ml of pva glue in the pound shop the other day. Or use old emulsion paint you might have lying around, which is easier to use and lets you see where you've been.

Reply to
Stuart Noble

brush it down properly and use water instead.

NT

Reply to
meow2222

More wisdom from someone who's obviously never done any plastering

Reply to
Stuart Noble

Erm! 1litre goes a long way. And yes you are supposed to dilute it. :-)

Reply to
The3rd Earl Of Derby

plenty of people do it. But maybe youre right, plastering never occurred before the invention of pva. what a waste of time.

Reply to
meow2222

Re-5Ltr going a long way. I just wouldn't expect to ever get around to using it again as I don't envisage any future projects that would need it (he said naively). Don't want more clutter

Reply to
John

It goes mouldy over time too.

Reply to
marvelus

You are IMM and I claim my five pounds.

Reply to
Weatherlawyer

But the bricks where new then. ;-)

Reply to
The3rd Earl Of Derby

My 5 litre container developed a mouldy crust. My wife wasn't overly impressed when she found me decanting it through a kitchen sieve.

Reply to
deckertim

Strange. I've had a couple of 5 litre containers for probably ~5 years, and neither has gone mouldy or lumpy.

Reply to
Andrew Gabriel

I think they used to soak the walls till they removed to suction and took many months for the plaster to dry out but as lime takes ages to set properly it didnt matter. Not acceptable today.

Reply to
marvelus

Water makes sand and dust motile. Wetting a wall before applying plaster to it also helps the mixing process where the alkali or base mixtures can get around small dust layers on a wall.

That is why you wet the scratch coat on a job you started the day before or whenever. You can add pva and that too helps the mixture the same way that Feb or surficants like fairy liquid will.

Another way wetting helps is in slowing down the rate that water s taken out of the mixture applied. This is why brick bonding can often fail in the summer where the bricks have not been sufficiently wet before being laid.

(Yes I do know that trouble can be introduced to the mix by adding detergents such as fairy liquid.)

Putting a layer of PVA on a brick wall and letting it dry out is only an expensive time consuming way of accomplishing what watering the wall would have done in the first place, allow the dust to mix in the mortar and slow the water loss through the brickwork.

If you wish to do that you will not damage the wall. Where it is used primarily is on finished plasterwork where no amount of wetting will give a plasterer a chance to work his magic.

This PVA layer stops any mixing taking place. There is a recent thread here about concrete and cements, discussing catalysts that cause them to set faster. Using water tainted with the previous mix to make the next will cause finish to go off sooner than normal too.

It's a problem in the first applications but a trick used to speed up the final working later in the day by the old school.

Another use for PVA is to make some fillers more pliant. It slows them down and it can be used in finish to make a small amount of filler or bonding in emergencies.

It may be used in a grout for self levelling cement for the same reason.

Finally, I was told a long time ago that you can bond plaster to glass if you splash a little 10 to 1 PVA on the glass. Why anyone would, escaped me at the time as I was so impressed with the statement I never gave that point much thought.

IANAP but I do know what I am talking about here. You must take my words for what they are and also treat some of the other posters here with similar circumspection.

Some of them are just chancers. Why they get upset when they have their ears slapped is beyond me. How else are they going to learn not to be silly?

Reply to
Weatherlawyer

But instead of allowing

it provides a barrier that will soften on the water side and bond to both the old surface and the "new one to be" and it encapsulates the dirt and dust. Thus slowing

I thought I'd better explain that a little more thoroughly before the fools start barking at my tyres as I run them over again.

Reply to
Weatherlawyer

The lack of an effective sealer was the reason a lot of original plaster and render was live from the start. If you had ever actually done any plastering, or spoken to any halfway decent plasterer, you would know this. Whether you like it or not, there have been improvements in the last couple of centuries

Reply to
Stuart Noble

TBH I'd rather you didn't start again, but pva was such an innovation, and is part of so many proprietary products that I guess it's an important issue.

I don't see how surfactants and pva have anything in common. The precise use of surfactants improves the properties of the mortar by reducing gaps between particles and the amount of water required, which results in a stronger mix. Plasticisers such as lignosulphonates work in much the same way. Indiscriminate use of either introduces water sensitivity into the end product.

Pva is nether expensive nor time consuming but I agree it may be pointless if all the bricks have the same porosity and the mortar is in good shape. If the porosity is unknown or variable then pva has the effect of regularising it.

Fillers such as Polyfilla contain their own binder (which is possibly pva powder anyway). I've never found any modification to be beneficial.

Quite right. I observe the materials I use and draw conclusions from my experiences with them. If others disagree I'm happy to be proved wrong. What is not IMO a valid argument is "do it this way because it's always been done this way".

Reply to
Stuart Noble

Well I've not used pva on any of the work I've done here and have had no problems. And I'm far from the only one.

NT

Reply to
meow2222

I guess it depends what preservatives are in it.

NT

Reply to
meow2222

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