why not PVA?

[spenny]

Hi guys,

ive always been lead to believe NOT to use pva glue as a primer, but have recently finished working with an experienced plasterer and tiler who used it to seal ply on our floors!? What Is the reason people dont reccomend using it? Is it just as bad or worse to use pva glue to seal walls?

[david campbell]

pva doesn't absord into the walls but sits on it,therefore when you tile your tiles and adhesive are being held on by a thin layer of pva

check out the thread
p.v.a v primer

[Dave]

This might be useful to you to read....and show your plasterer/tiler friend..


http://www.tilersforums.co.uk/tile-a...s-primers.html

[lucius]

Hi Dave i know about Ettringite but if a reaction occurs between cement based addys and gypsum plasters why isnt their a reaction between plaster and sand and cement renders?
Andy

[Dave]

Andy....it seems to be that when the sand cement has cured enough to add gypsum plaster to it the portland cement has hydrated before skimming...

But to add portland cement to a gypsum background is then when you are likely to get the reaction. if you see what i mean...

[lucius]

Gotcha, has anyone ever actualy experianced this.
Andy

[Dave]

Not personally but it does happen....so for a few pennies isn't worth the risk, don't you agree...

[david campbell]

http://www.tilersforums.co.uk/tile-a...l?nojs=1#links

gotcha dave

cheers m8!

[doug boardley]

Hi Dave i know about Ettringite but if a reaction occurs between cement based addys and gypsum plasters why isnt their a reaction between plaster and sand and cement renders?
Andy

a while back i phoned British Gypsum about this, and it's as Dave says!

[lucius]

Totaly agree Dave i always stick to the spec many of my customers supply their own tiles but i always insist on supplying the addy so i know what i am working with.
Andy

[LM Ceramics]

agree with abouve if you read the pva v primers then that all you need to know as soon as i read i threw the pva out of my van straight away

[whitebeam]

Do find that when plaster onto sand and cement gets damp/wet it can break down badly and starts to crystallize. That would be enttringite I assume

[Dave]

Do find that when plaster onto sand and cement gets damp/wet it can break down badly and starts to crystallize. That would be enttringite I assume

More than likely Efflorescence......

[FreeD]

I was also wondering about this...as rapid set is cement based...i do alot of float and set work where i'll skim onto sand and cement next day while still damp...this is ideal for plastering. Must be something to do with it being the other way round.

[faithhealer]

I was also wondering about this...as rapid set is cement based...i do alot of float and set work where i'll skim onto sand and cement next day while still damp...this is ideal for plastering. Must be something to do with it being the other way round.

wondered that one myself

[lucius]

Exactly you always hit the render with the finish the next day for that reason, i built a porch a while back rendered it out on the Friday with the intentions of going Saturday to skim but never made it in the meantime the client thought it would be a good idea to dry the render out by leaving doors and windows open i cant tell you what was coming out of my mouth Monday morning.You can get over this by putting waterproofer in the render to stop the suction this but stops the building from breathing bit like full tiling a bathroom loads of condensation.
As for the reaction between cement addys and gypsum plasters ime beggining to think it is an urban myth created by the suppliers to make a bit more money no one has come back with any first hand experiance as yet but i will still keep working to spec.
Andy