Originally Posted by carl h.
Dye the whole fretboard black if the lines are bothering you.
Might work. Depends if the fretlines are wood veneer (which is good) or plastic or epoxy (which is also good) or wood filler (which is bad).
Veneer or plastic fills the slots and gives the neck it's proper strength. The wrong wood filler won't and leads to neck troubles. However, veener or filler may take the dye easily while epoxy or plastic won't.
You can (as someone suggested) just coat the whole thing with black epoxy. Will work fine but will change the tone (to more Jaco like)
If you really, really love the bass and really really hate the lines, I'd just replace the whole fingerboard with a blank one. You can get one pretty cheap already smooth and pre-radiused from luthiers supply. An ebony one would really make the bass sing! Only the better pure black ebony is no longer available so if you want it pure black you'll have to dye that one too!
There are instructions online how to use a steam iron to take the old fingerboard off and you just glue the new one on and trim around it and tweak it in with side dots and sanding. I've done two fretless conversions this way and it wasn't that hard. Just be slow and careful. Or you could pay a luthier to do it for you. Results should be excellent.
But I'm not sure that lines would bother me enough to go to this trouble just to get rid of the lines. I've got a couple basses with lines and I'd prefer they were blank, but hey, *I* ordered one of them that way as a fretless noob, and the other one was used so it wasn't my decision. They both play just fine.