McP77

I think tiling on tiles in a bit of a personal choice. I don't like doing it as you just don't know what is going on behind the tiles. For example, what condition is the substrate in, how much coverage have the old tiles got, what the total weight will be etc etc.

With regards to removing the tiles, I find the easiest thing is to remove tiles, if the condition of the wall behind is no good, rip that off too and then dot and dab some plasterboard to the walls. You then have a near perfect surface to put the new tiles on.

As for the sand in the primer I don't know the answer to that, sorry.