To get paint to stick to and not flake off delrin and other slippery plastics: prime the parts with NAPA’s Rubber Bumper Primer or as it also may be called: Polypropylene Clear Primer. The part # is: 7223. It stays flexible and it seems whatever paint is put on top of it also stays flexible. I’ve literally tied delrin handrails into a knot whithout the paint coming off!
One caution, the primer contains laquer thinner, so don’t spray it on styrene.
A water base flexible paint is FASKOLOR. It is intended for Lexan model car (RC) bodies. There are lots of colors available, so any railroad color can mixed and matched with a little experimentation.
Based on your suggestion to “not spray it on styrene”, I’m assuming it comes in a spray can. Do you know if it can be purchased in a can or bottle so I can paint it on with a brush?
As far as I know, it is not available in a brushable form, however you can spray a bit of it into a container (paint Jar) and then brush it on, I do this much of the time. Use laquer thinner to thin the primer (the primer evaporates quickly)and to clean up your brush.
Be sure to completely cover the part or area. It works best when the object is completely encapsulated. It goes on a bit thick, but dries to a very thin coat. Details are preserved.
The primer dries quickly (5-10 mins) to a clear flat finish.
BTW The Napa stores may not have it in stock, but they will order it. It usually just takes a day or two for them to get it.
If I remember from the short time I had to serve managing a body shop, no it is not avalable in a form designed to be brushed on. It is available in quarts and possibly pints but formulated to be sprayed on. It has been a few years so I am not going to guess about the cost.