I had a small patch of filler on the tailgate of my first MGBGT. It blistered in winter with pimples. Eventually I used abrasive compound and it flatted nicely. But guess what? the next summer when it warmed up, what had been pimples became dimples. Micro dimples, which levelled when it was cold. That's how it stayed. It was localise to the filler which must have had water inside it.
