Clear coat powders are as durable as colored powders. Powders are available in different chemistries based on the application and expected exposure and environment. An exterior TGIC polyester type powders should work fine for you. Clear is clear and you'll see right through to the metal so keep in mind any defect or stain will be amplified. Heating the metal can change its color (darken) and chemical treatments for corrosion prevention and cleaning can also alter the metal's appearence. Using clear over another colored powder coat will add thickness and change the gloss level as desired. If the powder coat gets too thick it'll chip easier so too much of a good thing ain't so good.