When you are preparing a wall for tiling, it must be clean, dry, grease-free and flat. Screws, nails and any other projections must be removed and cracks filled and smoothed down. A previously painted wall is a good basis for tiling, but if the paint is flaking, blistering or damaged, it’s best to strip it off. Gloss paint should be scored to provide a keyed surface to ensure proper adhesion.
If a surface is already tiled, but you want to change it, you can strip off the old tiles. This is a long and dirty job, and once started there’s no going back. You’ll need to chop out one or two tiles with a chisel and then prise off the rest. Wear protective goggles to protect your eyes from flying shards of tiles. Hack away any old adhesive that remains.
Alternatively, to give a new look to old tiles, you can paint them (using special tile primer and paint) or tile over them as long as the old tiled surface is completely flat. Check for flatness with a spirit level. In either case, the surface needs to be sound, clean, level and grease-free, so clean it carefully with white spirit.