What works for me very well is a method I use for many years now, without any flaking. Even soft plastic bayonets who bend keep their paint on. After degreasing I throw a batch of 30-50 figures in a bath of 1 part white glue and nine parts water. This mixture is homogenized as best possible by stirring. I leave the figures overnight, after wich I leave the figures to dry, wiping away with a brush the excess white glue solution. Then I spray on the figures Humbrol 1, an acrilic gray spray primer. This spray primer is the best from several I tried.
Bob, I should probably have said, that now I use only acrylics for painting. I have not tried mixing enamels and acrylics on the same figure, but from information over the net, acrylic is ok over enamel, but not the other way around. A few years ago I used to prime with a brush the Humbrol 1 enamel. But I switched to spraying the same primer, but in acrylic. Much quicker with same results, or even better, as it is easier to get a thinner coat with spraying.