Simple green max works well. I would try using a damp sponge and wetting the surface to the simple green can get at the primer, and then use a good stiff toothbrush to scrub off the offending area.
Don't dunk the model in it, or you will have to redo the entire thing.
If you have any ridges, apply some GW clear coat to help the rough edges before you prime again.
But yes, it is going to be a bit of a pain.