Following the advices here, I rubbed my old F80 and D70 bodies with pure Isopropyl alcohol (99.8%) and a lint free cloth, and both recovered the original grip and look. It's a job that one has to do with plenty of time and detail to get a perfect finish, but it's worth the effort. I have a AF-S DX 18-70mm zoom that needs the same treatment on the focus grip. Just be careful to do it in a well vented room (Isopropyl alcohol is harmful if breathed). Thanks a lot for your help!