You can leave the paint on drywall provided there are not several coats and the tile epoxy will adhere with no problem.
How to install ceramic tile on painted wall.
Previously painted textured drywall prep.
Start by removing any molding switch plates outlet covers and other hardware attached to the area being tiled.
Sanding the existing paint helps the tile adhesive stick to the wall.
To install ceramic wall tile use a level and measuring tape to mark the tile locations on the wall with chalk.
Cover the lath completely with the mortar creating a new surface for the tile to adhere to.
Place the level horizontally on this line and.
Use a paintbrush to paint the edges and hard to reach areas like corners first.
Apply even pressure to force the air out of the thinset and to make firm contact between the layer of thinset on the tile and the layer on the wall.
Patch any holes or cracks with spackling compound.
Spread the mortar over the walls using the flat of a notched trowel pushing the mortar through the lath and onto the painted blocks.
Scrape away any loose or peeling paint or wallpaper.
If you have a large surface to paint use a roller for covering the tiles more quickly.
Spread a smooth layer of thinset on the back of the first tile and press it firmly into the notched layer on the wall.
Allow the mortar to dry overnight.
Use long sweeping motions to apply mortar to the wall and start placing your tiles in the marked locations.
Measure up from the bottom of the wall the height of the tile.
Check the tile for plumb and level then proceed to the next tile.
If the wall was painted prior to wallpaper installation follow the instruction for painted or textured walls.
Add another 1 8 inch to the height measurement and place a mark on the vertical line.
Once dry sand any high spots to create a level wall.
Remove any remaining dust oil or grease with tsp tri sodium phosphate or an ammonia based cleanser.
Likewise if you have plaster follow the instruction in that section.
To avoid any overlapping brush strokes start in one corner and work your brush strokes or roller down the tiles then across the area or wall.
Add water to the mortar while mixing until it s the consistency of peanut butter.