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Paint bubbling on galvanized chassis

I hope you will be able to take a minute and perhaps answer my question. We manufacture transportation equipment. All of our steel chassis get a phosphoric acid wash and then a phosphate coating is sprayed on it. We prime it with an epoxy primer followed with a urethane paint. We recently started to have some steel parts and even a whole chassis galvanized to increase its life span. We have run into what appears to be a gassing problem. After we have the chassis painted bubbles appear some big, some small. We are sure that the gassing is coming from the zinc coating. These are the steps we take, #1 we acid wash the chassis to remove any oils and contaminants. #2 after about 12 hours we apply an epoxy primer 1-1.5 mils. We bake the primer @ 140 degrees f. for about 40 min. #3 we paint the chassis with 2 coats of urethane paint and again bake it at 150 degrees f. for about 45 min. Almost immediately after it is painted the bubbles start to appear. We have tried a couple of things, one of them was to have the chassis sit around for about a week or so to see if the gases will escape. This has not helped, we want to offer the galvanized chassis as an option to our clients, but we need to figure out how to stop the bubbles from appearing first. Any ideas?

Stan Gansekoele
- Guelph, Ontario, Canada
2006

I have seen this problem a few times on galvanized. I am presently involved with coil coating where lots of galvanized is used. This is most notable with hot dip galvanized. We simply heat the coil after leaving the cleaner section prior to prime to drive any absorbed water from the zinc.

Ronald Zeeman
Coil Coating - Brampton, ON, Canada
2006