Surface preparation and the conditions for CUSTOM MBP primer over a painted substrate.
Do not assume thinset can bond directly to painted concrete. Paint can interfere with the bond, and the tile assembly can fail if the paint releases from the concrete.
CUSTOM Building Products does allow its MBP Multi-Surface Bonding Primer over painted surfaces when the paint is firmly bonded and the surface is thoroughly cleaned of contaminants. Do not use cleaners that compromise the paint. The finished assembly can only be as secure as the paint’s bond to the substrate.
This is a product-specific allowance, not approval for every primer, mortar or coating. Follow the current instructions for the selected primer and mortar. If the coating is loose or its suitability is unknown, resolve the surface preparation before setting tile.
Sources: CUSTOM MBP FAQ (page 1) and CUSTOM Technical Bulletin TB23.