What adhesive and grout advice can you give for my 4x4 Styron polystyrene Shower tiles? They are falling off, and I want to redo entire shower so they stay on for at least twenty-five years. What adhesive and grouts are best? (Worst?) Also, it's tough to clean old adhesive off the tiles. Any advice there? Can I use heat gun safely on these (thin) plastic tiles? Many thanks in advance for your replies.