Depends on what color Alclad it is. Chrome will need a black background, and I would assume that black plastic is the same as black primer. As for the other colors, I've sprayed several of them without priming and they look and dried just fine.
I've sprayed it onto bare plastic numerous times without problems. If you don't flood it on, it's no different than any other paint. Not saying it won't eat plastic, but if you do things correctly it isn't a problem.
Yep...but I'm also paranoid. I think the black layer is only necessary for the hi-polish chrome and one other color that I forget. But I never forget the primer.
Dan
"Laugh while you can, monkey boy!" -- Lord John Whorfin
The deal with spraying it on the black paint was to get a nice smooth surface. A high-gloss paint will get you the best (smoothest) finish job.
If the plastic is polished smooth, you can get the chrome look. I did that with the internal "nubs" that go inside the domes of the PL TOS Enterprise. Came out looking like chrome, which gave the reflectivity I wanted.