School boy error

[White Room]

I've seen tiles on a gypsum plaster no primer and they remained for many years, yes they should have been primed though...

 

[cone]

I've seen tiles on a gypsum plaster no primer and they remained for many years, yes they should have been primed though...

I prime and scrape all my work but what is the technical info on why primer is needed. How is it tiles stay on the wall when primed and come off easily when not primed.

 

[Rich Midge]

Crystals form between a wet cement based mortar and dry plaster. Bizarrely, this doesn't happen the other way hence why you can skim over render. By no means a chemist but this is the best I can do!

 

[Stef]

As Rich says, you will end up with ettringite crystals forming with a cement based adhesive against a plaster substrate.
Priming will give a chemical key but will also create a barrier between the two to stop ettringite forming.
I prime everything I tile, plaster, plasterboard the lot.
Read the guidance on your bags of addy & it will tell you whether to prime or not.

 

[White Room]

I prime because of the high suction from plaster, the cement based adhesive losses the moisture to quick to allow it to cure probably.
Was informed from a bod from Mapei that ettringite is formed when moisture/damp is present.

 

[Dash J]

Bang on the money there white beam
Suction

 

[Rich Midge]

It was the guys at Instamac that informed me of the crystals, this is going back 14 or 15 years. Though I agree that suction is a major issue I can't see it being the be all and end all. A damp sponge across a skimmed wall would surely prevent the addy from skinning over or drying out too fast but wouldn't increase the overall bond. Somebody somewhere needs to experiment?!

 

[Dash J]

I prime because of the high suction from plaster, the cement based adhesive losses the moisture to quick to allow it to cure probably.
Was informed from a bod from Mapei that ettringite is formed when moisture/damp is present.

I think suction is the be all
In a laymans term you are stopping the substrate from being so pourus by priming

Plaster,no primer,suck then pop de bonded

If it was a case of crystals then surely if you primed bonding plaster which is probably one of the most pourus substrates then you could tile it!
Me personally I would never tile bonding plaster if it had umpteen coats of primer

 

[Rich Midge]

Agreed bonding plaster will wick away moisture at a rate of knots but is only an undercoat plaster so doesn't have the inherent strength to support tiles regardless of priming or not. No addy manufacturer would guarantee their products straight onto bonding. Like I said earlier, I'm certainly no chemist just expressing an opinion. Personally I live by the prime everything rule.

 

[Dash J]

Agree mate prime everything