The coating applied to the substrate should protect against premature corrosion. The integrity of the coating can be assessed with respect to porosity and remedial work carried out if required. Porosity in the coating can be caused by numerous abnormalities in the coating such as:

Pinholes in the coating caused by air escaping from the surface of the substrate through the coating during the curing process.
Uncoated areas especially prevalent on corners or weld seems where coatings may have flowed away leaving the peaks of the profile exposed.
Cracks in the coating caused by excessive wet film build creating pressure during the flow stage of the curing process.