No worries, it all comes down to choosing the right paint for your project. Krylon for Plastics is amazing stuff, it actually bonds to the plastic rather than just sitting on top.
The metal stuff is good too, but you usually have to rough the surface to make it stick well. Which is fine if you have a sniper rifle, Just rough it up with 400 grit and spray on 5-7 thin coats. Then a few clear coats and it should last ages.
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