Don`t do it!!!!!!!!
take them off and put new ones up! we had a rented cottage, bathroom was cold and horrible with terrible tiles, they wouldn`t replace them so we got permission to paint them, we used proper tile paint, good stuff, cleaned the tiles with loads of suger soap/ don`t think we primed them, didn`t say we had to on the tin, in fact says no primer required, anyway left them to dry, and within 6 months where the shower fit the wall the paint was bubbling, as soon as a little hole appears you get this big bubble filled with water between the tile and the paint skin.
paint and you will need to redo the whole lot every year at least! and it only looks good for 3 months.
well thats what we found