Tried with various paints I had handy
Rust-Oleum crystal clear Enamel - Laser vaporized some of it, but no amount of repeats would get the last bit off the copper and it wouldn't etch.
Home Depot brand? Interior/Exterior Royal Blue fast dry spray paint - Lasers right off. Or looks like it does. Still leaves an etch proof invisible film on the board. Kind of pretty. Might make for good plaques and signs.
Krylon brand black paint pen - Laser barely did anything. I turned it up until it started burning but couldn't get a clean etc.
Acrylic "Ceramcoat" black paint - Didn't preserve details and dissolves in etch.
Green Sharpie - no discernible etching with laser
Black Sharpie - could see etching but not get all the marker off
blue painters tape - Oh, wow. Why didn't I try this first!?
blue tape.jpg