Q: I'm laying "penny" mosaic tile on top of a heating system. I had to use a light amount of thinset in some spots due to leveling issues. As a result I have some tiles that are coming up (I have not yet grouted). I am going to use spectralock epoxy grout (which I have used before) and know it dries very hard and strong. I'm curious if the grout will hold the tile, or if I should glue the tiles to the floor before grouting. Any input would be helpful.
A: No, The grout will not hold the tile. If the tiles are not properly bonded they will eventually come up. Make sure you use the proper thinset. Many porcelain and glass tiles do not bond well with standard setting mortars. I have seen this over and over again in my inspection practice, and almost always, it is due to the improper product. I would also tap the remaining tiles to make sure they are well bonded. If you hear a hollow sound when tapping you could have a bonding issue.
Fred Hueston, Chief Technical Director, Stone and Tile PROS