I know this is an area of some controversy so I thought there might be some interest in following up a DIY project I discussed back in 2012 here: Tiling onto plywood - https://www.tilersforums.co.uk/threads/tiling-onto-plywood.56767/ Well, seven years on with the shower in almost daily use there have been no leaks and all the tiles are still stuck firm . . . even the ones on the ceiling above the shower. The only issue has been a few bits of discolouration/mould in the grouting and also in the silicone bead where the tiled walls meet the shower tray. I'm expecting the silicone bead to be easily removed and replaced but having to remove the discoloured grout is more of a pain. So, two questions for the forum: 1. How long do 'mould-resistant' grouts typically remain mould-free? I used a Mapei grout - are there better ones for this or is there some sort of additive or after treatment that can be used? (but not those horrible 'whitener pens' that just try to cover up the problem!). 2. What's the best method to remove hardened grout? I'm thinking about using my Fein multitool with an appropriate blade but am open to any better suggestions.