QUESTIONI'm remodeling 3 bathrooms in an older home. The existing marble 12x12 floor tiles and grout are still like-new. There is no shrinkage or cracking. They were installed with mesh over the plywood sub floor.
What product should be used as a primer/ bonding agent so we can install the new porcelain flooring over the marble floor?
ANSWERANSWER - As long as the existing marble floor is structurally sound and well bonded to its substrate without excessive deflection, you can bond over the marble.
The marble surface is likely polished or honed and it might have a sealer on it or contaminates from pass use. I would recommend first scarifying the surface of the marble. This will take off any contaminates that could compromise the ability of the porcelain tile to bond to it.
You can apply a liquid applied waterproof/crack isolation membrane over the marble, particularly if there are any non-structural cracks in the surface or if you want a moisture barrier. But it may not be necessary.
You should use a polymer modified thin-set mortar adhesive that meets ANSI A118.4, or better yet ANSI A118.15, to bond the porcelain tile to the prepared marble surface. There are flexible thin-set mortars that are recommended for bonding tile over tile too. Make sure the thin-set mortar you use is recommended for this application.