I like my mosaic that I am using for my tub surround so much, I decided to use it for the vanity backsplash instead of the 3 inch marble backsplash that came with the vanity sink. My question is, what do I use to adhere the mosaic to the painted drywall? My mosaic is glass and metal. I already have Schluter All-set mortar and Ardex x77, but I have heard some people use a mastic. What should I use and do I need to sand or otherwise modify the painted wall?