Dear All,

I am sorry - I guess I'm looking for some friendly advice and I feel a bit stupid too. I am an amateur but have done quite a lot of tiling for myself, friends and family over the years.

I am re-doing the bathroom and I bought Mapei pro flexible cement based adhesive for the wall tiles. I haven't used cement based adhesive before but the guy in the shop said it was far better than ready mixed.

I read the instructions and it said use primer if going on to bare plaster. I just couldn't understand it 'Why' I thought - I've only used ready-mixed before when it is fine to go straight on to plaster so I didn't prime. Incredibly stupid but there it is!

Well now the bathroom is 3/4 tiled and now I know why you must prime - because the cement based adhesive reacts with the gypsum.

I thought 'Okay, you idiot, you've just made a big and costly mistake take them off and start again'

BUT these tiles are now really, really stuck on. I knocked off a couple today just to see and it will be a truly massive job to get them all off. Actually the adhesive bond to the plaster is greater than to the tile. There is no shifting the tiles and the adhesive without destroying them and the walls with it.

I am tempted to leave them, prime the rest and hope for the best. Does anyone out there have experience of this. How long does it take for the adhesive to react with the plaster? Weeks, months, years? What are my chances of getting away with it?

The walls are old backing plaster (1960's) + new skim to half way up the walls as the original skim came off with the old tiles which only went half way up). I only mention this because the plaster (except for a very thin skim) isn't brand new and maybe that makes a difference.

The tiles are Brtish Ceramic standard whites 148 x 148 + glass mosaics strips.

I know I've been an idiot but would really appreciate your thoughts and opinions.

Thank you. Sincerely, Ben

Hi and welcome....the reason I prime is the suction of new plaster pulling the moisture from the cement addy.

I did ask a adviser from Mapei about the reaction between new plaster and cement based adhesives and was told it would be if the two get damp/wet perhaps at alater date then a reaction could take place.

Thanks v. much for that mate, that's a comfort to hear - been depressed all day - the guy at Mapei told me that I'd have to knock them all off. Yours. B.

Don't beat yourself up Ben, the chances are you will be ok, the tiles are light weight so won't be overloading your walls. The one area that will be of slight concern is around the bath/shower where water is most likely to find its way through to the adhesive and wall. There are people who have done a lot worse and got away with it, at least you've used a cement based adhesive as opposed to ready mixed, that will go in your favour.

if there well stuck just leave them, but in future allways prime with cement based addy,

The moisture from the adhesive mix is enough wetting of the substrate and can cause Ettrignite... this reaction isn't an instant thing and can happen weeks after the installation..

As to some replies above.. would you not prime Anhydrite floors..? both are gypsum and both have the same risks..

OP, you realised your mistake and have help your hands up, yes you might get away with it , so as they say you learn by your mistakes... Gypsum undercoats and screeds DO need priming and priming properly...

Guys, Thanks for all your replies. It is genuinely appreciated that a person can write and ask for help and actually gets complete strangers saying 'what you need to do is...' Many thanks!... and I've primed the rest with two coats Mapei primer and will not make the same mistake twice. All the best, Ben