Hi folks! Does anyone know if I can use the standard grouting material to seal a tiled tub? (The tub is lined with cinder blocks sunk into the bathroom floor, and tiled with 12" marble tiles. Obviously if the tub is to hold the water, the grout is going to have to be waterPROOF, but the grout boxes I've seen seem to suggest that they might only be WaterRESISTANT. Is there another product I need to look for?