Thanks for report, Loris, sorry it didn't work out. It's funny how surfaces can fool you.
I've been experimenting with those ceramic floor tiles today. I was right in thinking that they would coat nicely, no problem at all with the coating -- even and smooth and no fisheyes. But the hardened gum won't stay on the tile, just floats off into the water.
My next idea, from a guy at my favorite local art supply house, is to try a Golden product called "Fine Pumice Gel," to provide more tooth for the hardened gum. I'm thinking of this rather than the gesso- pumice blend you guys have already developed, because I want something that won't obscure the surface and color of the original tile, and this is supposed to dry clear. I'll let you know how it works out.
I looked at alcan's US site and couldn't find anything that fit the same description as the Forex, but if it didn't work for you, it's a moot point anyway.