Krylon fusion is reasonably good, but you are limited in color choices. If the color you want is available, just clean the plastic well, and have at it.
I'd wash it with soap and water, and scotchbrite the surface to get down to something solid (if it is chalky), and wipe down with denatured alcohol.
Another option, is to look at how plastic bumpers are painted.
Sem makes a chemical cleaner, and also a flexible plastic spray primer. On top of the primer, you can use any paint you want, that is flexible enough to be used over plastic.
Actually, if it were up to me, I'd use the plastic primer, and brush on an exterior latex house paint on top.