Tacky PVA

[Richard C]

Glad it's only half of DIY plastering
Didn't expect this much debate on tacky PVA - good discussion!

RC & Mic; your doing an absolutely sterling job which makes very interesting reading; hats off guys & keep it up.

This sort of thread is perfect for debate which was, I believe, the sole intention but I could see it coming from the sister thread. This could go on & on & on; but for me it’s a bit like sticking pins in your eyes & so I’ve purposely kept out of it.

 

[roughcaster]

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.

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.

Micilin has said it all. It'll be like skimming/plastering onto a sheet of polythene, or a shiny painted wall. I wouldn't chance it,, ever, and i don't know how Bogan has got away with it for so long. I don't know whether to believe it really,,, i'd need to see it,,,, how does the plaster stick onto a shiny, unkeyed surface?
To be honest, i'm amazed that at what i'm reading here. Have a look at all the old posts,,, they all say plaster onto pva while wet/tacky, unless it is to kill off suction. In many of Joe's old posts, even he advises to, pva and "skim/plaster while tacky",, have a look through many of them and see.

 

[roughcaster]

Glad it's only half of DIY plastering
Didn't expect this much debate on tacky PVA - good discussion!

RC & Mic; your doing an absolutely sterling job which makes very interesting reading; hats off guys & keep it up.

This sort of thread is perfect for debate which was, I believe, the sole intention but I could see it coming from the sister thread. This could go on & on & on; but for me it’s a bit like sticking pins in your eyes & so I’ve purposely kept out of it.

I'm with you now Rich',, i'm saying "no more" after this post, but the majority on here would advise anyone else reading this topic,, DON'T ever plaster onto dry PVA.

 

[joe-90]

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.

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.

Micilin has said it all. It'll be like skimming/plastering onto a sheet of polythene, or a shiny painted wall. I wouldn't chance it,, ever, and i don't know how Bogan has got away with it for so long. I don't know whether to believe it really,,, i'd need to see it,,,, how does the plaster stick onto a shiny, unkeyed surface?
To be honest, i'm amazed that at what i'm reading here. Have a look at all the old posts,,, they all say plaster onto pva while wet/tacky, unless it is to kill off suction. In many of Joe's old posts, even he advises to, pva and "skim/plaster while tacky",, have a look through many of them and see.

I do say that and do do that but PVA hasn't been around as long as plaster - and the old boys managed. Most modern plasterers apply the PVA too thickly when its really more to do with suction than bonding. That's why we are hearing of so many occasions when paint won't stick due to PVA bleeding into the skim coat.

 

[Micilin]

I do say that and do do that but PVA hasn't been around as long as plaster - and the old boys managed.

How do you think they managed, joe?


Most modern plasterers apply the PVA too thickly when its really more to do with suction than bonding

Most? Too thickly? When it's more to do with suction than bonding?

Where is this all coming from?

 

[Third_Eye]

PVA the night before Plastering ! Try applying pva and once "dry" then for a test, sprinkle some water on the "dry" pva and watch what happens.

What happens is the PVA turns back to tacky pva ! The same happens with plaster when applied to "dry" pva.

Therefore why apply plaster when pva is tacky, when plaster makes the "dry" pva tacky ?...

 

[jrplastering]

i know this is an old post but i have only just read it and i have to say that third eye is right pva will liven back up if you apply moisture to it i have done it loads of times especially when i know the pva will take ages to suck in and it does liven back up once you apply wet plaster to it.
i diddnt believe it wen i first herd it when i was being taught but i did a little test patch (many years ago) and it does work.
i have never ever ever ever had a call back for plaster not keying .......infact i dnt think i have ever had a call for a complaint or work that is wrong......only calls asking to do more work.
i dnt think anone is wrong or right in this situation everyone has different ways as long as the work comes out good and everyones happy at the end...........and it stays up on the wall or ceilin

 

[SpunkyTheMonkey]

hmmm, ok here's a question.

we have a paint here called hydrochroma...it's pretty similar to dulux supermatt afaict. it has a pva based co-polymer, and zero stain resistance and washability (not sure if supermatt is the same in that respect), ie: if you wet it, it goes back into solution and starts washing off of the walls. when it dries, it feels slightly rough to the touch.

this is only ever applied to rendered walls, most of which have a reasonably rough finish so very good mechanical key.

would you guys pva that first or skim it as is?

 

[external]

hydrochroma sounds faintly like limewash,you need a mechanical key sealers wont get enough grip!

 

[SpunkyTheMonkey]

it's not limewash, it's very cheap pva based trade matt.

where a wall has been painted with that stuff it has a good key (skimmed walls are never painted with it) the question is whether it needs PVAing or not if you want to skim over it. conventional wisdom would say yes but my gut says no. it's not something i normally have to worry about so i'm wondering what you guys think since there must be people here who've done this both ways...

 

[Third_Eye]

I would say it is a MUST to PVA over the "hydrochroma" if its same as Supermatt...

Most Fillers Fall Off when used on Supermatt ! Now there is food for thought !

 

[Draughtsman]

This is good but approx. how long does neat pva take to become tacky after application?

 

[vinn]

The thread is seven years old, and from the bits I read its pretty confusing.

What do you find "good"?

What exactly do you intend to apply Pva to for what purpose?

"neat pva" is virtually never applied as such - its watered down.

 

[Draughtsman]

Thermalite and concrete block inner leaves, prior to render & skim. I'm just trying to find out, beforehand, what prep the spread should be doing. Oh, and some mesh on a concrete block inner leaf that's been crack stitched with Helibar.

 

[Doggit]

how does the plaster stick onto a shiny, unkeyed surface?

Most people think that glass is smooth, but the reason that it gets misted up, is that there are microscopic indentations in the surface that the moisture adheres to. So what you see as a shiney unkeyed surface, is nothing of the sort.

Having said that, if the plaster gets wet, walpapering etc, it can come off if there's only a single coat of PVA underneath.