Sorry, Steve. Whereas you might be an excellent painter and I respect you as a person, your advice on ceramic tile is at best questionable in some instances. Understand that I am not posting to personally challenge yourself, but instead, merely trying to prevent this gentleman's expensive ceramic tile installation from failing prematurely.
I think that because Luan plywood is commonly used as an underlayment for vinyl flooring that somewhere along the line somebody thought they would try it for a ceramic tile installation (maybe when nothing better was available). It should be noted that Luan plywood is not reconized as an approved substrate for ceramic tile by the Tile Council of America or the Ceramic Tile Installers of America.
Luan is not a suitable substrate for ceramic tile, period!. In fact, there is not a portland cement based thinset mortar or any organic mastic on the face of this earth that allows for this type of installation. The next time you purchase either product- READ THE DIRECTIONS!. It will clearly state that it SHOULD NOT be used to install ceramic tile over luan plywood. Why? It certainly isn't a bonding problem- it's a movement problem. Luan swells in the presence of moisture and expands and contracts both vertically (the worst) and horizontally. This movement can cause the tiles to pop up from the floor. Not to mention the grout problems that will also accompany this type of installation.
But don't take this advice from me or others that frequent this board. Educate yourself by viewing the following links: