Wow...I always get a little leery when I hear someone wanting to do this! I hope to God your not doing a FLOOR! Wall-tile is OK to do ONLY IF you use a Shellac-base primer such as BIN by Zinsser co.
** Then use 2 coats of a Semi-gloss paint.
** Use a good quality roller for the primer & Paint.
** Throw the primer-roller away. You CAN'T clean it after using this primer.
** I hope you're not painting in a shower-stall itself! NOT good results usually.
Thanks!
Faron