Usually pre-existing surface sealers must be removed. The best method is to use a paint stripper that can be bought at any hardware store. Use plenty of dry white cloth when removing the old surface sealer. This is particularly important with unpolished porcelain tiles which often arrive with a factory applied wax coating. Make sure you do a test area first. The exception to this is that a STAIN-PROOF^® impregnating sealer will pass through virtually any other impregnating sealer.