Plastic Paint Works!
Conventional spray paints just won't stick to plastic. Now paint manufacturers offer paint just for that application. These paints don't just stick; they fuse with the plastic surface to form a super-strong bond. Krylon Fusion for Plastic and Rust-Oleum 2x are two common brands.
Light Coats and Patience Prevent Runs
The only sure way to avoid runs on a vertical surface is to spray on a light coat and give it a little drying time before the next. That's hard to do if you spray the surfaces in random order, but easy if you have a strategy. Start by coating each vertical surface. Spray lightly to avoid runs. Then hit the horizontal areas before starting the second round. Repeat each round in the same order. That way, each surface will get maximum drying time before you return to it. If any vertical surface still looks wet, stop and remember this: An extra five minutes of drying time now is better than sanding out runs and respraying later.
Elevate your work (inset) Don't set your project directly on a workbench or newspaper; the paint will glue it to the work surface. The best way to prop up wood furniture is to drive screws into the legs.