Tried Simple Green ad IPA , can only scratch some off with nails. What is the next step?

I got a few lovely models that I would like to paint, but unfortunately, they were rather poorly built. Thus I need to strip the primer and fix them first.

They have been sitting in a jar filled with Simple Green, sealed for months, the paint should come of f easily. But, for some reason, the primer, that I have no idea which brand it is, will only come off if I scratch it with my nails, or very very brutal brushing with a toothbrush.

Clearly, that isn't any ideal for a miniature, which leaves me thinking what I should use next. My last resort is would be sanding and scraping, but I would like to avoid that.

Thank you.