I pulled the drum out the naptha in the paint tin to have a look. Here are the results after less than 24 hours. No brushing. No rubbing. No nada. Just rinsing a couple of times in the naptha bath.
I put everything back in the naptha to let it all steep some more. It will be a bit yet before I have time and space to move on to the next phase. Despite the excellent work of the naptha, I suspect that extracting and cleaning the spring will still be messy.