any tips on painting plastics?

[Bongo Prophet]

4. You could order online if not in a big hurry. Better to do it well than twice.

Considering the fact that your pieces will be used hands-on aggressively, there's little chance that it won't flake off. Even cleaning it won't help enough, though that is also a must.

If you just can't manage to use a paint made to fuse with plastic surfaces, your best chance is to sand the individual pieces well to get a good bite, then a good primer followed by light coats - and give clear coat at the end.

At any rate, I would do just one as a test first, and use it for a while.
Other paints stick better to fusion than the plastic itself, so it can be used as a kind of primer.

Fusion is what people who refurbish old commodore c-64s to be used as synthesizers, as an example. The right paint for the job.
Good luck, and...post pictures!