Rattle Can Blues

That is weird.  Usually enamel is well compatible with enamel.   It sounds like the black did not adhere well to the plastic surface.  Do you know what sort of plastic it is? Is this transparent plastic?

BTW, for the next time, aluminum paint actually stops light from being transmitted through it better than black does, surprisingly.

Walmart stores sell several brands and types of paint.  Which ones exactly were you using?  Were they enamels, acrylics- what?

Yes, I would advise not using Wal-Mart paint or any other hardware store paints.  Better to use paints designed for plastic models only.  Tamiya, Model Master, etc.

I would suggest different paint. That stuff sounds like crap.

if you are painting the interior which won't be seen so the interior lights won't make the hull translucent, wouldn't a coat of gray primer and/or a couple coats of white paint be solid enough? i have a lightship i may light the hull interior and was only going to use white primer.

I feel your pain.

I've never used the Walmart paints and I'm not familiar with their properties, but, given the week-long cure time for the base coats, my guess would have to be the top coat's the problem. Did you mist on the white gradually, or try to lay down a solid layer all at once? Even with compatible paint types, misting on the top layers as paint layers get deeper isn't a bad way to go. (Easier to do with an airbrush than a rattle can I'll grant you, but not impossible.)