Early FMs (as will many early run diecasts) will do this and yes, heat and humidity will accelerate the process. During the tumbling process, a solvent is used. It gets deep into the pores of the model. If not cleaned properly, which was the case, this solvent tends to leach out over time. It opens the pores further and causes the metal to go funny. It's like aluminum oxidizing. A few years ago, while at FM, I had addressed this with an engineer and they resolved the issue with proper cleaning of the Zamak. To avoid this issue, its a good idea to put a coat of wax on a model. Let is sit out for about a month, then wax it. Working with GMP and Meguiar's we developed a Care Kit. The wax seals the paint and prevents any issues from "popping" up. For cars that have this problem, you can try polishing the area. If it is too deep, the paint will blister off. Then, the only solution is a repaint. I have a friend who does this for a living. Later cars should have this problem resolved. I know that the Maroon '41 Lincoln is notorious for this and it starts as fine paint distortion. This is a great stage to stop it. Hope this helps. BTW, if you personally need a Care Kit, Gregg, let me know.