if you look back at Sheldon's reply earlier in this thread, he advised that you apply tung oil after the sanding/filing. An oil finish is very thin and is worn away even by your fingers over time. That's why Sheldon recommends monthly waxing of the neck if a bass is played a lot. Sandpaper cuts right through the oil down to bare wood very quickly. Think of it this way: you were removing seams in the wood, not seams from the oil finish, right? If you got rid of the wood seams by sanding, you removed wood [i:zhudwqy0]and[/i:zhudwqy0] the finish. You've got go through the finish to get to wood. If you didn't apply oil after sanding and filing, then those spots are unfinished and prone to swell even more now, not to mention they will discolor from dirt, oil, and sweat from your hands.
Mark