Hi,
Applying coatings under variable atmospheric condition always presents challenges. Grit blasting, you know, is always wise -- both for an adhesive anchor-pattern and for rust-oxide removal.
Your choice of an epoxy, zinc-rich coating sounds ideal. If its failure, loss of adhesion, seems premature, take a closer look at your environment. Then speak with your coating manufacturer.
For example, how UV stable (ultra violet light) is the coating? Is atmospheric temperature exceeding the thermal stability of the coating? (Depending on color, black for example, there is always the 'black body effect'.) What about flexural strength of the coating, compared with the steel beneath?
My expertise is typically associated with preventing wear, friction, corrosion or sticking -- mainly indoor, dynamic environments:
Nevertheless, look to the following companies and products for help:
Heresite Protective Coatings
Wearlon Coatings (now known as Environmental Coatings)
PPG (their version of PVDF, polyvinylidene fluoride)
Best wishes,
William Gunnar
Industrial Coatings World
William Gunnar