Hi all
What’s the best way of removing mastic from tile joints, has become discoloured, will replace with waterproof grout
Rake it out or use some form of solvent

You can get sealent remover from most DIY stores and it works pretty well. Squeeze it on, leave it for a couple of hours and scrape off.

Silicone remover to get rid but I would replace with decent bathroom silicone like Dow Corning 675 and not grout, tile joins need to be able to move or you will get cracking.

WD40 is your friend and a lot cheaper than sealent remover, doesn’t take long either. Try it.