Even with a painting handle, it's absurdly easy to rub off paint from the corners while working on these things.
What paints do you use? I like Lifecolor a lot, and they're very soft, with the issues you describe. On the other hand, Citadel paints - particularly bases - set rock solid. I prefer the thinner nature of the Lifecolor ones to do details, but happily use Citadel for that reliable base.
And please, don't mention anything that comes in the abominations that are dropper bottles.