Hi there I have built a shower stall with 19mm wbp plywwod and have tiled it with 10"x8" tiles. I have two questions should I use grout or silicon in the corners where the tiles meet? and should I use an admix additive to the grout or seal the grout after it is up ( the grout is water resistant) the stall has three tiled walls and hinged door? I have used silicon between the tile and the shower tray?