QUESTIONHow do you waterproof a tile floor that is already installed? - trying to waterproof 18 x18 inch ceramic tiles on bathroom floors. spills are passing through to the floor below.
ANSWERANSWER - If the ceramic tile is already installed there is no way to fully waterproof the floor after the fact, unless you replace the tile and install a legitimate waterproof membrane during the replacement process.
Some people will try to put a sealer on the grout joints, but that doesn't waterproof it. It doesn't work. It only makes it more water resistant.
The only thing you could try on, on an experimental basis and if you deem it practical, is to remove the grout joints by cutting them out with a dustless diamond cutting wheel on a grinder. Clean the joints out thoroughly and fill them with an ASTM C920 sealant (caulking). This type of caulking is either a full strength silicone or a urethane sealant. Be sure to also caulk the perimeter of the tile where it runs into walls and tubs with this same sealant. If the tile is impervious and all the joints are filled with this type of sealant, you might end up with it being waterproof, depending on how good of a job you do. Good luck.