Re: Sanding my J Custom neck.. have questions....
I had the same issue with some necks I've worked on. I though I had them really clean and as soon as I oiled them, it brought up some dirt. On the Jem DY neck I worked on, I just tried to strip it using naptha, ammonia and steel wool, but never actually sanded the neck. It looked super clean prior to oiling. It also to this day still smells a bit of patchouli from the previous owner.
When I did an FP neck that was much dirtier to begin with, I sanded the neck after stripping, then went to the oil/rub out/sand to try to get rid of all that deeply ingrained dirt and nastiness. It seems to have done the trick, because the neck was fairly badly nicotine stained and had a nasty stale cigarette smell when I got it and the smell is totally gone. The maple never got as bright as new, really white maple, but it's dramatically improved and totally even in appearance.
This thread prompted me to put my RBM2 into my rotation. That neck was also never sanded, but it's pau ferro so you can't see anything. It had been rubbed out previously with steel wool, but it still didn't feel totally smooth and in spots seemed to have some small lumps. I rubbed the whole thing down with an old sheet of 6000 grit Micro Mesh (which is in reality much coarser than 6000, more like 3000). You can clearly see where my fingers were as the paper picked up some light surface oil. Now the neck has a nice light sheen and is totally smooth.