remove any loose and naff tiles. but what i would be inclined to do is treat the tiles aswell, you can do this by getting a 4/1 mix of pva and water dip your brush in the pva then whilst it is loaded dip your brush in sand (any sand will suffice) then paint the tiled wall. do this the day before you dot 'n' dab and you won't get a problem. the wall when dry will feel like sandpaper to the touch, an ideal key.
A professional tiler once told me that there was no better surface for tiling on than old tiles. Mind you, he didn't dot or dab. He put his tile adhesive on with a toothed spreader.
What you want mate is a product called "Thistle Bond-It" by Brithish Gypsum the only place I have ever bought it is In "Hall & Rogers" but im sure you can order it anywhere its a sort of thick pva solution ready mixed that has a sort of latex feel to it and has small plastic beads in it so when it drys its like a really heavy sand paper finish which is the perfect key for plastering over no need to dot and dab cheaper to do and no loss of space in the room, I have used it and believe me it is amazing stuff it really is.
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