Hi, I'm retired, not a plumber but a keen DIYer. I occasionally help a mate who is a one man band - he is a qualified sparky and has taught himself domestic plumbing. His main line of work is refurbishing bathrooms and I labour to him for a couple of days. We constantly disagree about applying tile adhesive to the walls prior to tiling - he "back butters" every tile which I think is mad, he says applying the adhesive directly to the wall doesn't work as it all falls off (??). We normally tile directly onto freshly erected plasterboard. Can someone help me convince him he's wrong?