Even a factory paint job on a tank is not perfect. Most of the time it's done by a guy spraying it by hand unlike cars which are done by robots and come out perfect in completely dust free environments. There is usually overspray, spray on rubber parts, and lots of masking issues. Just saying tank paint doesn't need to be perfect and by the time you weather the tank afterward it may not even show up anyway. I had an effect like that happen when I used an oil based paint over an oil based primer on my track motorcycle fairings. I switched to a different primer and the paints didn't interract anymore.