I erase my PMs as soon as I act on them to keep the directory open - fills up pretty fast and is of limited size.
Lab Metal is similar to bondo in function but different in chemistry. It doesn't use a catalyst like 2-part bondo does. It hardens by evaporation of solvents only.
I'm 'expert' in powder coating like I am 'expert' in roc 'n roll music - great audience material!! Not sure what would do that but since the coating is the same plastic all the way through and the stain you describe sounds like a superficial surface blemish, can you just polish it out? What does your vendor say about it?