Prime using a good drywall sealer. I remember years ago painting some new drywall without priming and literally having a gallon of paint soak up into one sheet of rock. I also suggest doing a bunch of research on tile and sheetrock. 25 years ago I tiled over sheetrock and now have to remove it and put in a better backing surface.

Casey
"Sean McDonagh Racing"

I believe that it is not recommend to prime or paint drywall that will be tiled over. That being said, if this is a wet area you should not be tiling over drywall at all. You either need to apply a waterproof membrane of some kind over the drywall, or put up cement backer board.

I didn't comment when I saw the first comment come out because when it comes right down to it, either way is fine. The reason it's usually recommended against tiling over painted surfaces is that you can't predict when the paint will let go, and the only thing I, as the tile contractor can warranty, is that the thinset won't let go from the paint. Now, with drywall, I've yet to ever see paint over drywall, UNDER THINSET, let go. This is why I don't have a problem tiling kitchen backsplashes that have been painted for a couple of years. However, I also don't have a problem setting directly over bare drywall, either. You DO have to be watchful, though, that the thinset doesn't dry out on you or skin over too quickly.