Tacky PVA

[NickB_99]

Hello all

Reading Richard C's comment about PVA being a plastering aid (//www.diynot.com/forums/viewtopic.php?t=226968) prompted a question - but I did not want to hijack the thread, so a new one.

Hoping this is not a daft Q....but
Widespread knowledge that you should skim when the PVA mix is tacky.

PVA reduces suction is a given, but does the tacky PVA greatly enhance the bonding of the skim to the substrate?
Or perhaps is it that tacky PVA aids in allowing the skim to draw off the trowel?
Or anything else?

Basically, is there a combination of effects here or only one?

 

[roughcaster]

I would say there was a combination of the two, but with the main purpose being to bond the plaster to the substrate.

 

[trowelmonkey1]

agree with RC, as always never considered the fact that it would help the muck off the trowel though newb, good insight!

 

[bogan]

Is it a given that you should plaster when PVA is tacky? The main reason that i have plastered onto tacky PVA is that it is the soonest opportunity to get on and spread the gear. Especially when spreading onto a bonded wall I like to PVA it the night before as i find it gives you more troweling time when the PVA is dry. Also it means I can get prep done the night before and start spreading first thing in the morning, very handy during the dark winter months.

 

[joe-90]

If plaster sticks to a plastic bucket without PVA it'll certainly stick to just about any substrate. The PVA simply stops the water being sucked out the mixture and little else.

 

[roughcaster]

Edit.

 

[roughcaster]

Is it a given that you should plaster when PVA is tacky?

Yes,, of course it is!!!! Try plastering onto a dried out PVA'd wall/ceiling,,,, or rather... DON'T.!!!!!!!!!!!!!!,,,,, ever.

 

[Micilin]

If plaster sticks to a plastic bucket without PVA it'll certainly stick to just about any substrate. The PVA simply stops the water being sucked out the mixture and little else.

Try squeezing a new plastic bucket with plaster in. It will flake off.

Over time, the scuff marks on your mixing bucket provide a key that the finish sticks to, the finish does not bond to plastic. A thick coat of finish on a dirty bucket , when squeezed or banged will fall off, taking most of whats stuck to the bucket with it.

Plastering on to dried PVA is something you should never do , it is a classic beginners mistake - unless you have provided a mechanical key by making a slurry. Glue will not stick when already dry.

 

[joe-90]

Yes I agree with that but a brick/block doesn't have the finish of a new bucket.

 

[Micilin]

Yes I agree with that but a brick/block doesn't have the finish of a new bucket.

Obviously, but you don't put PVA on brick /block to form a key. They have a physical key (PVA in this case is for suction control, not adhesion).

You put it on a smooth surface ie one that does not have a physical key so that the plaster will stick to it with the chemical key. If you let it dry, the glue has set , so you need a fresh coat. 'Just about any substrate ' won't cover it, I'm afraid

 

[joe-90]

What substrate won't it stick to without PVA?

 

[Micilin]

What substrate won't it stick to without PVA?

Substrates such as existing finish plaster and painted surfaces - perhaps half of DIY plastering?

Edit- Doesn't have to be PVA , but you need something to do the job (like WBA, Bondit )

 

[bogan]

must say I am a bit confused by this. I have plastered onto dried out PVA loads of times and have never had a problem with it. The smoothest surface that i plaster onto is plasterboard and obviously never use PVA on that.I have read comments from all of you and am in no doubt that you know your stuff but on this point i can't the reason why you would need to stick the plaster onto any surface. for me its just suction control.

 

[NickB_99]

Substrates such as existing finish plaster and painted surfaces - perhaps half of DIY plastering?

Glad it's only half of DIY plastering

Didn't expect this much debate on tacky PVA - good discussion!

 

[Micilin]

must say I am a bit confused by this. I have plastered onto dried out PVA loads of times and have never had a problem with it. The smoothest surface that i plaster onto is plasterboard and obviously never use PVA on that.I have read comments from all of you and am in no doubt that you know your stuff but on this point i can't the reason why you would need to stick the plaster onto any surface. for me its just suction control.

Finish chemically bonds to the plasterboard, so no need for PVA (maybe old or sound block boards )

Obviously , if this is the smoothest you come across, that's fine -when you have to plaster on existing flat plaster , painted work etc?

Look across the light at a flat smooth wall with dried PVA, and see how shiny it is. Not a good surface to try and stick to. Very risky.

I would go home rather than try it, honestly.