I'm currently in the middle of building countertops for my new kitchen cabinets in preparation for 12" x 12" porcelain tiles. I currently have 3/4" and 1/2" plywood sheets glued and screwed together and fastened to the cabinets as the base for the countertops. Unfortunately I didn't do enough research beforehand and bought regular plywood as the substrate instead of exterior grade. I was initially just going to apply Ultraflex 2 tile mortar directly to the plywood and then stick on the tiles. I figured that if all the grout lines were sealed correctly and regularly maintained that water intrusion would not be an issue. I assumed this because my parents recently changed their kitchen counter tops from 6" white gloss tiles to slab granite and when my dad pulled off the old tile and mortar himself (to save on labor cost) we noticed that it was just regular plywood underneath. Their house is ten years old and there were no signs of water intrusion.
Anyway, it seems that after some research this won't be acceptable (especially around the sink). I'm leaning towards using this Schluter Ditra sheet membrane that I've been reading about. Will this work with the thinset that I purchased? Another option is cement backer board but I'd rather stay away from this since it will be harder to install not mention raise the height of my counter tops and cause issues with my edge molding. Also, is there any paintable liquid membrane that I can apply to the plywood to seal it prior to applying the thinset?