Problems with PVA? A warning.

[jrb]

I'm a new member here, and I've asked for quite a lot of advice in the last week or so, so I thought it time I offered some in return. I'm not really sure which section I should post this in, so chose 'General Discussion', but if there's a better place, then mods please feel free to move it.

I've been reading some through a lot of older posts on here in the layout construction section, and it amazes me the number of posts I've seen where people are 'sealing' baseboards, backdrops etc. with diluted PVA, but then having problems further down the line with stuff not sticking properly & peeling off.

Now, I'm a kitchen fitter by trade, and that involves all sorts of other trades; plasterers, electricians, plumbers, tilers, decorators etc., and if there's one thing I've learned it's that PVA is a remarkable product, but only if it's used properly.

I don't know how or why it's got into the minds of the general public (and a LOT of tradespeople too) but trying to seal any surface with PVA is a really bad idea. When applied to a porous surface, PVA will soak in slightly, and then harden - as it's designed to do. But (and it's a very big but!) if it's exposed to significant moisture again after it has dried, such as more PVA, wallpaper paste, or paint, it will absorb some of the moisture from that and become 'live' again (basically it softens). Once this happens it doesn't harden again - ever.

So, if you've 'sealed' your MDF or ply or whatever backdrop with diluted PVA, let it dry, and then glued a paper backscene to it with more PVA, don't be surprised if it bubbles up & peels off! Same with gluing cork to baseboards too. I'm not saying don't use PVA for either of these applications, just don't seal it with PVA first.

Now, some of you may think that this is utter  , but it's not. Tilers used to recommend sealing walls with PVA before tiling, but every single tile adhesive manufacturer will tell you not to, and that doing so will invalidate any warranty. Decorators used to recommend sealing fresh plaster with PVA before decorating, but every single paint manufacturer will tell you not to, and that doing so will invalidate any warranty. Same with wallpaper adhesive manufacturers.

The only exception is that plasterers will sometimes use a PVA product to coat brick/blockwork before plastering, but that's a slightly different scenario - the PVA is applied & then the plaster put on soon afterwards, so the PVA isn't left to fully cure beforehand - it dries (for the first time) with the plaster. Also, the type of PVA used for this is a different formulation & is designed for that very purpose. Contrary to what some conspiracy theorists will tell you, it's not just PVA wood adhesive with a bit of water added and the price doubled!

If you really want to seal some timberwork, there are much better alternatives. Pick up a tin of waterborne matt varnish, or some sanding sealer. Or even some emulsion paint. All of those will do the job of protecting the timber, and won't be incompatible with gluing other things to them later on.

All of the above is, like anything on the internet, only the authors (my) opinion. You will find others agreeing with me, and probably an equal number saying the opposite. You'll even find some sites (like DIYdoctor, for example) that have conflicting info depending on which page you read! In trade circles this has been debated ad infinitum, and there will always be those people that won't agree. What can I say - they're perfectly entitled to hold their completely wrong opinion

Seriously though, even if you could use diluted PVA, why would you when a tin of varnish will do the job a whole lot better, and with far less mess?

[Railwaygun]

does this apply to "waterproof" PVA??

[jrb]

does this apply to "waterproof" PVA??

To a certain extent, yes, as it's not so much "waterproof" as "water resistant"

[johnlambert]

In some ways this is good news.  I've got some ballasted track that I need to lift.  The ballast was secured with diluted PVA (the track was glued with Copydex); if I understand correctly all I need to do is soak the ballast with water and it should unstick.

[Portpatrick]

Tips like these are one of the benefits of the Forum.  I have never used PVA to seal boards - I normally use a primer sealer, but I had wondered about PVA.  Not now though.

[Caz]

I´ve used diluted PVA glue to fix my ballast and whenever I´ve had to do alterations, a quick soak and the old ballast and glue removes very easily allowing easy reuse of the track.   

[Tank]

Great post, which will definitely be of use to many of us.

does this apply to "waterproof" PVA??

I've never had any problems with waterproof PVA, and I used it a lot as a floorlayer for filler when laying solid timber floors.  I've even used it on the top of my fenceposts in the garden before painting - to stop too much paint soaking into the ends.  I haven't got round to painting them in 18 months, but the glue is still rock hard, despite rain and the garden flooding!  Perhaps it depends on the brand?

[Zogbert Splod]

Yeah, I've had a problem with PVA...  Not the kind you are talking about though.
When my eldest daughter was just starting school they were doing a 'singing' lesson one day.  I had taught her that a certain nursery rhyme went 'Poly vinyl acetate, poly vinyl acetate, poly vinyl acetate, we'll all have tea...'
My ex and I were asked to attend a meeting with the head teacher where we were told, in no uncertain terms, that this was being seen as 'undermining the work of the teaching staff' as my daughter had argued with the teacher about who knew best - 'My Dad says...'
I couldn't keep a straight face and the head teacher got really angry when I burst out laughing at the whole episode.  We moved soon after that which is probably just as well.....

[TylerB]

Sorry for my ignorance, but what is the reason  for sealing baseboards?

[Malc]

Sorry for my ignorance, but what is the reason  for sealing baseboards?

Stops warping if the absorb moisture from the air. If you use prime them, stops glue soaking straight in when using scatter. Best to prime both sides to stop warping. I use MDF and MDF primer.

[EtchedPixels]

Not just warping. Most of the length changes in a layout are the wood changing water levels. Sealing it properly stops that. It also helps when you put PVA on top as the PVA won't soak in and make anything warp either.

I count the fact that hot water can remove PVA a feature not a bug - it's sometimes a saviour in times of dire need

Alan