I would go thinners but quickly. Wash it off frequently, and reapply, wash it off. Cotton buds to tease it off. It can take a long time, do it over a few days, weeks, a little at a time. A lot of what I do is like this, I am super careful, especially with old plastics. You may want to try alternative solvents, sometimes carburettor cleaner works better or may be acetone; just don't let any thinner or stripper sit too long, better often and rinse and take your time.