pva'ing before plastering

[Stephen]

Hello,

I have some areas of loose plaster, where the top coat has come away from the base coat. Is browning just another name for the base coat?

I think it has been mentioned here before about drilling holes and pouring PVA down to glue the two layers back together but I've just bashed off the loose plaster the good old fashioned way ;)

I've had a read of the wiki "plastering beginner's guide". It mentions using a dilute PVA mix somewhere between 5:1 and 3:1 water to PVA. The base coat is very sandy and I am sure that the PVA is very important to prevent the layers separating again. Are all PVAs the same? I am confused because TS sells so many: interior PVA, exterior PVA, and this:

presume that's the one I need but what are the differences between the different types?

I had a pro plaster my ceiling and he poured from a 5L bottle of Wickes' PVA but only added the slightest drop of water; far from being

5:1 it was more like 1:50! Why so strong?

I also found a post from this group where it is recommended to PVA conduit before plastering on top of chases. I've never done that before, should I start to do so? Does it give a rougher surface for the plaster to adhere to?

Thanks.

[Andrew Gabriel]

Browning is a particular type of scratch (base) coat. When I did my plastering training, I was told it's rarely used nowadays, and consequently, I've never used it. For the scratch (base) coat, I use either bonding coat (which works on everything except damp walls), or sand/lime/cement/waterproofer (for damp walls).

They 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).

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, and when time is money, buying it already diluted is probably cheaper).

It may have been already diluted, or he may have been after using it only for bonding (see below).

It is used for two purposes, often in 2 coats.

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. You apply this and let it soak in and wait until it dries (or at least, feels dry).

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) and you start plastering ideally whilst that is still tacky. This will effectively glue the finish coat on, and by using this technique, you can plaster just about anything that doesn't flex. It doesn't matter if the PVA has dried by the time you get to the last bit of plastering, as the water in the plaster will resoften the PVA.

It glues the plaster to the plastic.

[Stephen]

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]

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.

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?

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.

A dilute coat such as 5:1 acts to stablise the surface if it's

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.

This is what he needed to glue the plaster to the ceiling. That explains the difference, thanks.

[about conduit]

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?

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.

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. These worked for me as a complete beginner, but for people who can plaster, plaster is probably the way to go.

Thanks again.

[Andrew Gabriel]

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.

I suspect that means already diluted.

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.

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.

Eh? You don't use pure cement.

[Stephen]

So selling exterior PVA is just a way to part a fool and his money then?

[Wickes PVA]

All of their PVAs say ready mixed, so I guess some must be more diluted than others, otherwise why such a big price difference?

Sorry if I am being dumb, do you use the PVA diluted or straight from the bottle for this?

Cement is listed in the TLC catalogue, though perhaps that's for repairing around back boxes rather than filling chases?

I had a bit of cement and a bit of sand and some holes to fill so experimented with both.

I have used the Wickes ready-mixed mortar and it dries without shrinking and sands to a smooth finish. It's nice for beginners like me.

I tried homemade mortar but it was too grainy and needed a top-cot of polyfilla; perhaps Wickes use a finer sand? Cement on its own is not as nice to handle and impossible to sand if too much is put on.

For filling chases, since the conduit was just under the surface, I did not need to fill very deep, so I just used a couple of mm of interior filler. if I was more competent I would use plaster.