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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?


My recommendations would be to degrease, fresh water wash, apply a 'Mordant Solution' ie 'T' Wash. after chemical changes have taken place, Fresh Water wash, paint using a High Solids Surface Tolerant Epoxy eg. Carbomastic 15, or Interplus 256, or something of that generic type, apply 125 microns DFT. Follow up with a High Build, High Solids M.I.O.
Epoxy, to 125 microns DFT. Follow this with a Water Borne Urethane Finish coat 50 microns DFT. The forget all about it and eventually retire in peace without ever giving it another thought !!!!


As long as there are no oils on the surface - then power washing and priming the surface with an acidic wash primer - no matter how long it has weathered - would seem to fill the bill for a standard pretreatment of the zinc.  
The type of primer I'm thinking of is a single pack, polyvinyl butyral and phenolic resin based blast/etch primer containing zinc chromate pigment and phosphoric acid to etch the surface.  These go on in two coats that can be applied within hours, to about 0.5 mils DFT.   
Check this concept out with your favorite/most credible paint company.  PPG has a product call MP 176, but I've not used it specifically.

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