Non solvent based primer for acrylic paint.

I like to use a flat acrylic paint when I need a primed surface prior to appling my finish colors. The flats seem to grab better. You don't need to cover completely, just a light misted application...unless you are applying a light color primer over a dark plastic in preparation for say a white or yellow where you need a uniform base for better color coverage.

Polly Scale has the best adhesion of any of the acrylics I've tried.

And it's not that acrylics don't etch the surface—neither do most enamels made for plastic.

Enamels contain solvents that readily dissolve skin oils. Most acylics don't, even those that use an alcohol solvent.

The key to acrylic adhesion is to get the plastic surface CLEAN. 

Acrylics are lousy for primers because they lack the solvents... They don't etch and "grab" the plastic...

You might check the local hardware store for a rattle-can of latex primer though...