I am installing a new tile backsplash in my kitchen. I am using a 2x2 / 2x4 slate tile with a backer. My question is whether I need to use a mastic adhesive or if regular grout will do the job of holding the tile in place on the wall. I've commonly seen a mastic used, but usually with ceramic tile and I don't know if it can be used with a natural stone tile.
The walls that are receiving the tile are finished walls, do any of you recommend putting on a primer first or some sort of waterproofing? thanks for your help!
You can't use grout for installing tiles. Grout is for grouting, not adhering.
I would suggest removal of the finished wall behind the tile and installation of either a tilebacker cement board (wonderboard, durarock, hardiebacker) or at least some clean drywall.
Mastic works, but thinset works better. I'd personally opt for thinset over mastic for most applications. Dry mix works best, but you can buy pre-mixed tubs of it. Thinset would probably adhere to the paint, but installation of the backer would ensure it will.