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Welcome to the Bunnings Workshop community @Crackers1. It's sensational to have you join us and many thanks for your question about tiling over wet area plasterboard.
No sealer is required when tiling over a product like Aquacheck in an area that doesn't require a waterproofing membrane. The Building Code of Australia specifies which walls, floors, junctions and penetrations are to be treated as waterproof or water-resistant. However, if you are using it in an area that traditionally requires a waterproofing membrane, like a shower recess, then yes it does need to be sealed and that sealer would be your membrane. Here's the residential installation guide for Gyprock products. You'll find requirements for moisture resistant plasterboards on page 47.
Please let me know if you have further questions.
Mitchell
Hi @Crackers1,
Personally, I'd add a membrane behind the tiles on the splashback just to be safe. It might not technically be needed, but any water that sits on the bench and gets wicked into the grout will transfer into the plasterboard. The plasterboard will probably be fine with a bit of water as that's what it is designed for, but better safe than sorry.
I'd recommend sheeting the painted plasterboard. If you think about it, all the tile and adhesive weight is hanging off paint. If the paint lets go, so do your tiles.
Mitchell
