I'm familiar with Golden acrylics, and have used them on canvas for some time; top-notch stuff, but yeah, top-notch prices!
I had a similar problem a few years back as your door issue; it took a change of primer to nail it down. It seemed like there was a difference in densities of the styrene, as if it was not fully incorporated and had hard spots and soft spots. Krylon sandable primer, sanded smooth with 800 grit and followed with a coat of spray-can etching primer (I forget the brand) as a barrier coat, and the enamel paint (Testors red) laid down smoothly.