Tom,
Yes they are powdercoated, as a topcoat. For both iron and aluminum based materials the key to corrosion control is in the conversion coating and the primer system. Without the conversion and primer, the corrosion control is worse than a traditional liquid enamel ( cross linked or not ) due to the non permeability of the material accelerating corrosion. The reason for the change to powder was not one of superior performance, but one of enviromental and financial benefit for large scale production without a degredation in quality. From a pure performance perspective, you would be light years ahead to prime a fuselage truss with an epoxy liquid, vs just using a single stage powder.