Install stone back splash directly on painted wall or install plywood/cement board base first?

I'm installing a stone back splash behind my bathroom sink - 16 SF. The wall is currently painted and I want to make sure that the stone and thinset adhere properly. Should I sand the wall with 80 grit sandpaper and apply the thinset/stone or should I put 1/2'; or 1/4'; plywood down for better adhesion? Perhaps cement board?Install stone back splash directly on painted wall or install plywood/cement board base first?
Ideal way is to do a cement board as a backer board but since its in a non wet area,( not exposed to direct water) you really don t have to.Roughing the area up is always a good idea and won t hurt a thing. I d never do a wood backer in a moist and humid environment, It can expand from moisture and break the bond.


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