You're suffering from some misconceptions here, Jim.  The application of AB air brakes in place of KC equipment usually added enough weight that cars were reweighed when this was done, but not always; the stenciled tank car light weights apparently could vary considerably from actual weight.  As for repainting,  the photographic evidence suggests that GATX cars were repainted around every 10-12 years in the steam/transition eras, but there were certainly exceptions. However, repainting changed the weights so little that the cars were almost never reweighed after repainting unless other work (e.g., AB brakes, truck replacement) was also carried out at the same time.  It was not unusual to see cars that were 20 or 30 years old with "NEW" reweigh dates.  In fact, as an extreme example, I have a photo of a UTLX VanDyke tank car taken c. 1951 which still bore "NEW 4 12" as a reweigh date, despite the fact that its arch bar trucks had been replaced by much heavier cast steel integral-journal-box trucks.