I'm thinking it would depend on how the paint treatment is to be applied and where you intend to paint (eg interior yes/no?). Also, which model REO.
I'll assume spray application including interior on a Grand. With that in mind...
* remove the spring
* remove the bottle and tubing
* mask 510
* mask the delrin insert with tape (leaving it in gives you two flat surfaces for easier masking plus it will allow the pos contact to be covering inside the insert)
* rubber cement on the exposed metal feeder tube
* rubber cement on the magnets (both on the
mod and any doors you want to paint)
* mask the exposed firing button (remove any button covers)