I said some time back I was researching metal protectants. I forgot about it for a while, went back to my notes tonight and cleaned up some stuff. Here’s what I have, would like your opinion. I’d suggest looking at the Gibbs and Boeshield links before anything else.
Trying to minimize anything gunky, but want it to last as well. I think that’s the ultimate trade off as far as I can tell, you choose between lasting or gunky. The Gibbs may penetrate and last if you do yearly touchups, will be looking at it more. I like the idea that  it’s dry.

Nice info page:

Probably my number one choice at this time:

Number two choice:

Here for possibly useful future reference, will still be looking at these more for Mustang just in case, at least will look again before purchasing Gibbs or whatever:

Much better than average forum page FOR ADVICE:

Some quotes from this page:

Boeshield works, but the solvent for the paraffin wax stinks. FluidFilm is lanolin-based and doesn’t smell as bad when applied. It seems to not be as waxy as Boeshield.
It seems that Gibbs is not waxy. Boeshield has a paraffin wax base.

The Gibbs stuff looks pretty interesting, especially for long-term use on polished metals or stuff like magnesium. Seems that it dries and doesn’t leave a waxy or greasy film. Seems to me that I’d spray Gibbs everywhere possible, then POR-15 over that for most places that are accessible. If not POR-15, maybe Boeshield, but the POR-15 will last forever. In the sense that WD-40 was made to protect missiles in silos in the ’50’s, Boeshield was developed by Boeing to protect aircraft.

I think regardless of what I wind up choosing, I’ll also have to figure out:

1 – How much it will take to coat the bottom of a car.

2- How often I’ll be doing that (yearly, whatever).

3- If a sprayer will be okay if it comes in a gallon jug. Some of this stuff only comes in 12 oz spray cans (Boeshield if my memory is right).

Once I pick something, then I have to contact them with my questions.

Of course, I can’t think of the surface area on the bottom of a car as flat! I have to approximate what the actual accessible area is! All those dips and folds and protrusions, etc.