Just a thought despertaferro, but I suspect that the thinners within enamel paints are what is affecting the plastic of the figures you (and I) are having problems with. I am going to try sealing the figure first with a coat of white acrylic.
Right, well I have just undercoated a dozen Caesar figures with white acrylic. Will wait and see how that turns out. If it doesn't work, I may try Gesso.