We are developing noise-attenuated panels with 16 ga, G90 galvd, sheet steel skins that will become sound proof envelops for power generating equipment.  They can expect to be in service 30 years, and be used in any U.S. location.  At some future time we cannot control, the owners may field paint these panels with high build, high solids, polyamide epoxy.
Here is our problem.  We know some weathering will take place, but cannot define the amount during specification.  The enclosures are assembled in multiple locations, and each has multiple aspects, so assume the zinc on a continuum from passivated, to part weathered, to full patina. We need to specify a surface preparation, etch primer, and/or tie coat that can meet these conditions. We have the option to buy unpassivated steel.  However, assume sweep blasting is not an option, and any chemical treatment must meet EPA etc.  Can anyone help?