I had the same problems when I was painting. Fortunately, I'm now a full time sheetmetal shaper and don't have to worry with these problems, but anyway...When I was fighting this problem, we were using PPG K36, and although the filler was finish sanded with 320 grit prior to primer, we still got sandscratch swelling and bullseyes. We NEVER painted the same day as we shot primer. Always waited at least overnight and was assured several times that we were waiting plenty long enough. The folks at PPG assured us that 2-3 hours was plenty long enough to wait before topcoating. What a load of crap!! When doing restoration and hot rod work, I would wait at least 7 days before blocking primer, and never had problems, but you can't do that in a production shop. We fought it every way possible and still never got a handle on it. My opinion is that the base coat acts as a wick whenever fresh solvents are applied, such as spraying primer. I believe where the base is exposed due to featheredging the paint around the repair, that it sucks up the solvents in the primer and they become trapped for several days.
NOW, Just about the time I quit the body shop, I got in touch with Barry Kives at Southern Polyurethanes and we had a long discussion about this. At that time, he had a new product just hitting the market that was a fast cure 2K primer. 'Turbo 2K' I think is the name?? Anyhow, I used it on a Chrysler Voyager hood that had previously been repaired with DuPont products and still had the OEM finish. The only thing we were to repair was the rock chips, and there were several of them!! To give the SPI Turbo primer a real test, I sanded out the rock chips with 80 grit on a DA, followed by 180 and finished with 320. It should have been stripped, but what better way to test a product. We informed the customer going in that we were using it as a test vehicle and sould need to see it monthly for 6 months and if it failed, we would strip it and repaint as no additional charge. After spraying the primer, I let it set overnight, lightly sanded it with 320 on DA, followed by 600 wet. It sanded out beautifully. The paint went on super slick with no reaction to the primer and after three months, it still had no signs of sandscratch swelling. Barry assured me that the Turbo 2K primer was a 3-4 hour total cure product.
I'm completely sold on this product as well as many others I've tried from SPI.
Most primers on the marke have a 7-21 day total cure rate. The difference is monumental!!
Try it!
Randy