Is this ceramic tile? How are you adressing the valleys or grout areas which are concave if you will? I am afraid that after the paper has dried that the valleys will show up clearly and make your project not to your expectations. If ceramic tile, you might consider floating the tile to make it even and straight, but then I do not think any floating product will stick to the ceramic tile itself.
Good luck, I would definately suggest that you remove the tile first.