Tanking new plaster shower area. Does it need a mist coat before Primer?

[zenonithus]

Hi all, I removed the old skim coat in the shower area exposing the old sand and lime. Had the plasterer skim over that with some bonding. Should have got it properly skimmed really, though there's no time. I want to tank this area. I need to prime it first with the aquaseal tanking primer. Though the bonding is a little rough so should I just put the primer straight on it or shall I mist coat it first with 50/50 contract mat?

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[L95]

You shouldn't apply anything with Portland Cement in it for a minimum of 4 weeks over plaster mate. Runs the risk of ettringite failure but if you can find a tanking kit that runs without Portland, then that would be okay.

You'd be wasting money and time giving it a mist coat. If anything you'd weaken the load carrying capability of the plaster even further.

 

[zenonithus]

Hi L95, the sand cement is old from the 1920's and had bonding put over it. The tanking went on though cant help but feel it was a bit of a waste of time as the tiles would bond stronger to the plaster. Feel like with enough force one could pull the tile off on a rubber coating. Also definitly do not paint asyou said it will weaken the bond. The primer on the raw plaster then the tanking on that when dry.

 

[L95]

Hi L95, the sand cement is old from the 1920's and had bonding put over it. The tanking went on though cant help but feel it was a bit of a waste of time as the tiles would bond stronger to the plaster. Feel like with enough force one could pull the tile off on a rubber coating. Also definitly do not paint asyou said it will weaken the bond. The primer on the raw plaster then the tanking on that when dry.

Unfortunately, the plaster is the weak point. The moment you apply plaster coatings, it reduces the weight load of the substrate to 20kgs. Please don't tell me that you used ready mix over the tanking though..... I've seen that a few too many times and wish it was clearer on adhesive packaging.

 

[zenonithus]

I've actually not tiled over it yet. All I know is that it needs powdered plaster as that chemically sets rather than ready mix which sets by moister absorption through plaster. I'm finding the membrane peals off the wall at the edges though I think these were the areas where I painted and it is the paint that is lifting.