thinset over drywall?

I am soon to be installing a backsplash above a utility sink (small job). The backsplash will be a mosaic of travetine and fused glass tiles. The area is currently painted drywall.

The thinsets I have on hand all say "do not install over gypsum" which makes sense to me.

Question-- what type of thinset and/or mastic do I use?

Thanks

Do not install over "raw" gypsum. (As in raw joint compound)

Any modified thinset could be used for this application. Scuff the paint and tile it.

Any modified thinset could be used for this application. Scuff the paint and tile it

So he should prime it first?

(That's a joke only Bud will get. Do not prime your drywall. Just sand it.)

Thanks folks. Never been to this site-- I've now learned more than I ever want to know about lots of things & my boss certainly appreciates me spending my time this way!