Trying to strip the factory finish off acorn style center caps? What works?

I'm trying to strip the finish off a set of factory acorn style rally wheel center caps. I've been able to remove most of the finish by a lengthy process of glass bead blasting the center caps, however, the outer edge of the center cap and the center of the acorn has some material on it that seems impervious to even glass beads and chemical paint stripper.

What have other folks used to successfully (and easily) strip the finish off of the factory style acorn rally wheel center caps?

Once I've had these center caps stripped of their finish, I'm hoping to have the center caps powder coated in a silver color. I've already had the rally wheels powder coated in a rally wheel color and they came out great. I use these wheels on an open car trailer.