You have to use a plastics primer first. On GRP you might just get away with it, on modern plastic bumpers, no chance: the stuff they're made of is so slippy I'd almost be prepared to take a mold off them in GRP without using any release agent!
Definitely a plastics primer. 2K primer will flake off. If you can find out the material talk to your local auto paint shop and ask what they would recommend.
Putting your location into your profile can be useful. "ffggfgffg" suggests either you too lazy to do so or that you don't know. Neither is a good sign.
what sort of plastic? theres loads of differant sort.
most paint manufacturers have web sites that will tell you whats,what.
grp fibre glas must be self etched first then most normal primers are ok to use.
you can get self etch in spray cans. do not try to put to much on,just enough to cover.
does'nt matter if it's a bit see through.