It's kind of interesting that lemon oil can cause so much confusion.
The reason that many places discourage it is (including the Martin citation above) is that if you go to Home Depot and buy a bottle of lemon oil you're buying a bottle of solvent that has some lemon oil mixed into it so it doesn't smell like kerosene. Interestingly, if you put this on the finished parts of your Alembic (e.g., not the fingerboard), then the solvents will strip dirt off the polyester varnished finish and you'll probably get a pretty nice looking instrument that smells lemony.
But if you put that lemon-scented solvent on the fingerboard, it'll strip the oils out of the bare ebony making it look worse. If you have an old school nitrocellulose varnished guitar including Martin acoustics, vintage Fender electrics, or any better Gibson electric, then you may or may not have an unpleasant chemical reaction (which you can read as costly in collectable value and/or repair costs), so you don't even want to go there.
hydragyrum had a really good post about limonene (you should write Material Safety sheets!). If you go to Target, you'll see that there's a lot of house cleaning products that use orange or citrus oil as a green solvent. For a long time, I figured that they were using citrus oil to cover up the bad smell of some natural solvent, but it actually is the citrus oil that's the solvent. If it's as bad-ass as xylene, then you wouldn't want to use pure limonene to clean your instrument either, but I'm sure it's in fairly low concentration in extracted oil. Just because it came from a natural source doesn't mean that it isn't insanely toxic when concentrated. If you SO had a 50 gallon drum of nail polish, it would have probably had to be delivered by a guy in a hazmat suit!
When you talk about tung oil or linseed oil, these are finishing oils for bare wood. When you put that stuff on a piece of raw maple or mahogany, it soaks into the surface, then forms polymerizing bonds as it dries. This has the effect of making the surface much harder than the natural wood, hard enough to resist light scratching and take a bit of a shine when you polish it. And if scratch it badly, then you can just apply more oil and reharden that spot.
If you apply tung oil to a finished surface like the polyester varnish on your Alembic it will just sit on the surface and get gooey. The polyester finish will be pretty much impervious to anything natural or synthetic that you put on it. So, I don't think this is going to work either.
So what do you do? For the finished areas of the instrument, I think you can treat it like a car finish - a non-abrasive chemical polish or a very fine abrasive polish will work, then you can wax is with something that feels good after you're done. I bought some stuff at a woodworking shop from a company called Livos which is a liquid which is mostly carnauba which dries hard and doesn't feel sticky. I use a German wax called Klasse on my car which would be good too and is similar to Zymol waxes. Turtle Wax feels pretty sticky on the back of a neck.
The one thing that you DON'T want to use on your bass if you can avoid it would be a car wax that contains silicone. Silicone waxes are very resistant to environment and easy to put on, but the silicone is very difficult to remove from wood if you ever need to do refinishing, causing you to have to sand everything down way into the wood to remove all traces. If you have any silicone in the wood, the finish will look blotchy and won't adhere well. Body shops hate silicone too.
For the fingerboard (oh, was that the original question?), you want to use something that will clean the wood and hopefully not remove oils or damage it.
The reason that rosewood and ebony are used for fingerboards is because the act of playing a stringed instrument will abrade the fingerboard. If you put a varnish on the fingerboard, it will get ground off over time. You can try something like the tung oil here (and some instruments do), but it's probably going to degrade really quickly once the hardened part of the surface is worn through. The smart choice is to pick a hard, oil-rich wood like rosewood or ebony and leave it raw. It will wear from contact too, but there's no surface finish to wear through - it's this hard all the way through.
Some of you probably have played modern Musicman instruments (after Ernie Ball took over, which has been a long time now). They have maple necks and fingerboards with a gunstock oil+varnish finish, much like tung oil. I have a Van Halen guitar, and the problem is that the maple fingerboard gets pretty grimy looking (a Stingray would have the same finish). I called Ernie Ball to ask what they recommend and spoke with Dudley Gimpel, the guitar designer there. He said their recommendation was to use Murphy's Oil Soap to clean the fingerboard and neck. It's gentle and pH balanced so as not to attack the wood or finish. I've done this on most fingerboards and it seems to work pretty well. The only problem is that it's not a very effective cleaner compared to powerful chemicals, so a lot of elbow grease is required on a grungy fingerboard. They didn't have any recommendation for putting anything on after cleaning it.
The other thing Gimpel said was just not to worry about it - keep it clean so it doesn't have undue abrasive wear, and don't worry if it doesn't look like it did in the store. That's probably pretty good advice too.
So, I'd think about giving this a try and see if it works for you.
David Fung
