PVA Use as a Sealant for Substrates?

[jonny round boy]

I decided to start this thread as it's something I've often wondered about, and Peter Parfitt's latest video has just reminded me as he uses PVA to seal MDF before applying a water-based latex adhesive for his leather-topped desk...

I'm sure this is one of those age-old, never to be resolved arguments, but I've been told that you should never use PVA as a sealant on anything that is to have a water-based product applied to it (despite the fact that PVA manufacturers market certain versions of it as a 'building sealant').

The reason being that PVA dries fully only once. If you then apply water to it, either on it's own or in a water-based product, then it becomes 'live' again and will always remain slightly soft. I don't know if that applies to all PVA, or just 'normal' PVA (as opposed to waterproof PVA).

Plaster manufacturers tell you not to use PVA to size walls prior to plastering; plasterers tell you that's just because the plaster manufacturers want to sell you their own 'special' (and therefore more expensive!) sealing agents. Most plasterers will insist on sealing rough/dusty brickwork with a PVA wash before applying bonding. I've even know some instances where they've told clients to 'seal' fresh skim with PVA before painting it! I doubt very much that the paint manufacturers would concur...

I do know that every major tile adhesive manufacturer specifies in their warranties that the warranty is 100% void if any part of the substrate has been PVA'd, as the PVA forms a 'live' barrier which stops the adhesive from bonding.

I also know that most builders look upon PVA as some special concoction that they're happy to slap just about anywhere!

This is all (mostly) based on things I've been told by different people over the years, and very little of it is based on cold, hard fact. Most people have their own opinion on it, and in a way it's like religion - those that are sceptical cannot prove it doesn't work, and those that believe don't appear to need any proof that it does!

Does anyone have any input on this? I'm particularly interested if you can back up your opinion with facts!

Jonathan

 

[Stone Message]

Hi Jonathan

Some of what you say must be correct - my friend who restores antiques in his spare time told me that it was not necessary and I do mention that in the final video of the pedestal desk series.

As for plasterers, each of the three that I have used in this house have wanted old plasterwork sealed with weak PVA before applying new plaster.

I have used weak PVA to seal MDF prior to veneering (using PVA for that too) and it has been fine. The idea was that without sealing an absorbent surface the glue layer would dry too quickly. I have also used weak PVA on new plaster prior to using emulsion and it worked well.

I am quite happy to learn from experts on this but I have never had any issues after using weak PVA as a sealant.

Peter

 

[Davej]

For me Pva is a must when plastering over old surfaces.  In many years of re skimming I have never had any plaster blow and I have done a lot of ceilings as well as walls so I will continue to use it . For sealing prior to tiling , in none critical areas I use S.B.R.  In showers/ wet rooms etc I always tank the whole thing to provide a waterproof background before tiling as per regulations .
Dave

 

[Stephen B]

[size=12pt]
This is a popular product in Australia for many of the purposes discussed above. Some, including myself even add it to hand/shovel mixed concrete mix to promote 'plasticity'.

http://www.bondall.com/bondcrete/Bondall_Bondcrete.html

Is it, or something similar available in UK/NA?

Edit.
Should have stated that it has the look and feel of PVA but with a resin additive.