What I have found out with my ongoing research and testing of painting Tomy tracks and such and learning this from researching how to can spray automotive plastic bumpers is that the molds are sprayed with a releasing agent that keeps the plastic from sticking to the mold and helps to release the parts from it once the part has cured. That same releasing agent still lingers on the plastic part and it is this agent that prevents paint from sticking to the plastic. This release agent can be something as simple as a WD40 spray. So cleaning the part with mineral spirits or alcohol first is a must. My latest experiment with a piece of track so far is the best. and updates on the experiments will be posted here once I can come up with a procedure that works.
I haven't tried the Uni paint sticks as I have never seen them but will give them a look at. What I have are the Testors Paint Pens and Sharpie Oil Based Paint Pens which I like because not only do they come in a fine point but also an Extra Fine Point which is great for detailing.
