I've lost count of the number of cleanings I've performed on my QDL -- spritzing with lacquer thinner on sunny days in my yard -- but eventually I did remove decades of crud and it's working properly.
When dealing with gunk, persistence pays off. Eventually.
We've all grown up in an era when we're promised instant cures, instant answers, but sometimes you just have to doggedly attack a problem with the most likely solution.