I am putting ceramic tile in a kitchen. Presently, the subfloor is 5/8" plywood, 1/4" luan, and vinyl tile flooring. I want to remove the tile, put hardibacker, then the ceramic tile. I have two questions. One, is it okay to leave the luan? Two, is it necessary to put a mastic between the hardibacker and the luan, or can I just screw it down? Thanks.