You're always supposed to put a coat of clear powder over it from what I've read
It depends on the powder, some clear powders will actually dull the finish. the translucent candy colors will dull if a clear is applied, the same will happen to a hi-gloss color as well, how the powder reacts over a period of time depends on the powder too, there are 4 types, i have listed the 3 most popular;
A TGIC (triglycidyl isocyanurate) powder will have good mechanical properties, gloss retention and will cover sharp edges better but wont stand up to solvents as well as a urethane powder.
Urethane-cured polyester powder coatings combine very smooth, exceptional thin film capability, with excellent mar and chip resistance, and good weathering properties. Adhesion to properly prepared (pretreated) substrates will provide very durable coatings with long-term resistance to humidity and corrosion. Polyurethanes are typically resistant to many diluted aqueous acids, salts, aromatic and aliphatic hydrocarbons, grease, and oils, it also has a very good hi-gloss retention but its film thickness is limited and will be thin on sharp corners.
Epoxy coatings are used primarily as functional coatings for substrate protection where inherent toughness, corrosion resistance, flexibility, and adhesion are required. The primary limitation of epoxy-based coatings is poor weatherability.
the super chrome from caswell is a polyurethane powder but because of its hi reflective properties it requires a top coat to keep it from dulling.
So if you are having someone do your stuff ask them what powders they are using and make sure its right for your part.
