We do plastic bead blasting where I work and the flex additive works. I have people all the time asking me to remove paint from plastic auto bumpers. Plastic bead cracks the paint off and if it has flex additive it has a little "give" to the paint and will not come off well. We quit working on plastic bumpers, by the time the beads go thru the paint it kind of chews on the plastic and makes it rough. There are commercial can paints advertised as paint made for plastic, I have not used any so I can't say if they are good or bad.
Danger