If you only have primer on the model and no other paint then soak it in denatured alcohol. This will enable you to use a nylon brush or toothbrush on the the surface without marring the detail and panel lines. It takes some time but is achievable.

A riskier technique which I have tried and does indeed work (I stress you MUST BE CAREFUL) is to brush on lacquer thinner over the areas that the primer is thickest. The lacquer thinner literally melts off the primer. So the idea is to brush lacquer thinner on to the affected area with four or five strokes and then wipe the brush off on paper towel or a rag. On the paper towel or rag you can see how much primer is being lifted. Do not flood any area with lacquer thinner as too much of this solvent will eat away extremely aggressively at the plastic beneath the primer leaving you with a somewhat bumpy and melted surface. And by God man don't use this stuff anywhere near clear parts.

Better yet....stick with denatured alcohol.  ;D