Can I use Eco Prim grip to tile outside shower over exterior painted wall? If not, is there a primer I can use? Is grinding the paint the only way?

Thank you for your inquiry regarding MAPEI Products! Mapei cannot recommend any of our products over a painted surface. The formal recommendation is to grind off the paint from your surface and wipe clean with a damp sponge. Once clean and dry and sound, you can tile right over it using a polymer modified thin set mortar suitable for use outdoors. The Mapei Large Tile & Stone Mortar is the only exterior use thin set mortar Mapei offers at Lowes. It must be protected from rain for 3 full days and from frost for 21 full days after tiling. If you would like additional assistance with your project, please call us here at Tech Support Mon-Fri from 8 am to 5:30 pm EST at 800-992-6273. Thank you! Product Advisor # 11