I have a section of plywood below a frosted window (beside our front door) which I'm interested in tiling on top of. I'd like to do a nice design with various pieces of broken tile. I've done a fair bit of floor and wall tiling inside the house but I have a couple of questions about this new project. First, what kind of mortar/mastic/whatever should I use (it is an outside wall but it is protected from direct rain). Secondly, is it even possible to tile a design on an existing vertical wall with small tiles without them moving and falling?