A true "bare metal" finish is possible, but don't think that it's easier or simpler than paint; it's just a different kind of effort.
Like Jamesgs4 says, start with a pristine surface. If you want it to stay pristine, some sort of clear-coat might work. However, if you're after a slightly ratty look, just keep it oiled. Some folks keep bare metal wiped down with WD-40. I've met a few rat-rodders who swear by used motoroil, sometimes cut with a little kerosene. Works okay, but you have to keep it up, and you may end up with a slightly odorous surface.
My preferred brew is a 40/60 mix of beeswax and double-boiled linseed oil. Over very gentle heat, slowly melt the wax into the oil, just until it can be thoroughly mixed. Then just rag it on/off like any other paste wax, but hot. Helps if the metal is warm too (think "out in the sun" heat, not "under a tiger torch" heat). Sometimes you get a few streaks or globs; I just burnish those out a bit with steel wool. Go light and build up coats; if you go too far, it's not impossible to burnish it back down.
You'll get a little color this way, a very slight red-brown tint. Depending on how you apply and/or burnish, you may get some faint residual streaks (I like this look, but it depends on your tatset). Also, it can soften slightly if re-exposed to head (so don't coat engine parts with this, obviously). It might stay tacky for a day or more after application; you can speed this up by using boiled or double-boiled linseed oil, and/or adding a capful of Japan Dryer to the mix right before application. Unlike lubricating oils, linseed oil is "drying" and will eventually harden. The beeswax gives it body and lightens the color a bit.
Ultimately, it gives you a weathered, slightly-dirty looking bare-metal finish, one that looks like it's on the edge of rusting, but never actually does. Try a few test pieces, and work out the ratio of wax to oil that works for you.
and a kz
