Membrane Forming Sealers: This type of tile sealer will resist water. They are mostly employed as grout colorants with added pigments to change the original color of the grout. Membrane sealers should only be used on unglazed tile because they will not adhere to glazed tiles. They generally stay on the grout’s surface, forming a nonporous membrane with a glossy finish.
In high moisture areas like a shower, water will find it’s way to the tile/grout underlayment- using a membranous sealer makes it hard for the water to get back out and let the grout dry. This can be cause issues associated with prolonged water exposure, like rot, mold and mildew. So best to use this type sealer only with floors.