Painting Evaporators

[vendo81d]

What is the best way to paint an evaporator?  I asked a guy with Seymour paint and he recommended using their cold galvanizing primer, followed by Alumi-blast paint, and then clear coat.  I prepared my unit by sandblasting it at 40-60 psi and then cleaned it with wax and silicone remover.  Any parts that could be removed I had powdercoated.  Everything seemed fine unit it got more humid this month and now the paint is flaking off.  I don't think I got more than 6 months out of this paint job.  I'm extremely frustrated and now I have to re-do five refrigeration units.  I have attached a picture of the problem.  This is from a Vendo 56.  What is the best way to get a long lasting paint job on an evaporator???

[davestar]

Are these new evaps, or are they originals?  I'd be concerned about the amount of aluminum oxidation in the fin portion, if they were older units.  I'd look at some type of etchant, either an etching primer, or even a quick water bath with perhaps a caustic, and then a good rinse.  I wouldn't use Nitric acid though, it would be really bad for the Copper tubes.  I would then blow dry with good dry air to get entrapped water out, and then paint after I was sure I was dry.

Please note, I'm not a restorer, I'm a metallurgist.  I worked producing evaporators for an automotive company for a number of years, and I was responsible for their chromate/conversion coating process, which is similar but different from painting.  So YMMV.

[vendo81d]

These are old evaporators.  I ended up sandblasting them back to bare metal and having the guy who paints my Coke machines repaint them.  He used an epoxy primer and a silver top coat.  They look great now.  I have been running two of the machines for about a month now and so far everything is holding up well.