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If it is truly solvent pop then it should not come and go. Sounds to me like you have moisture in the paint. What kind of moisture trap do you have in the system? How often do you drain the water from the pressure tank? How many filters do you have in the air lines? What was the relative humidity during the painting process? And the temp? Solvent pop usually occurs within a day or so of painting, not weeks later. I would have color sanded the paint within 2 days of painting in order to help release any trapped solvent rather than waiting for it to "soak" into the underlying layers of materials. Like Redtractor man said, sandable primer should have a sealer sprayed over it just prior to painting. Forgoing that option, make sure you sure you sand the surface not more than 24 hours prior to sparying and use 240-280 grit paper. Dust it off very well and tack it down completely. Spray a mid-coat adhesion promoter just before the top coats are sprayed. This will help "glue" the paint to the primer. To determine whether the paint or primer is lifting wait until the pops show up and use a sharp razor blade to prick open the blister. Bare metal tells you the primer is the culprit and visible primer under the removed paint tells the rest. I would venture a guess and say that the primer is coming loose from the metal. Are you sure you used a good self-etching primer on the metal and old paint prior to the urethane primer coats? You might also have a materials compatablity problem. Theoretically all "dried" primers should be able to handle all top coats, but we know better than that.
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