Fluid Film is awesome for protecting undercarriages from corrosion. Fluid Film is formulated from specially processed wool-wax, highly refined petroleum oils and selected agents to provide corrosion control, penetration, metal wetting and water displacement
https://www.amazon.com/dp/B00B93ZXGS/?tag=fordraptorforum-20
I've used fluid film before on stuff like dirtbikes & snowblowers (would have tried it on my GM trucks but GM uses a protective wax from the factory so I just purchased & reapplied what was already on there: Daubert Chemical Co. NOx Rust or something - decent but requires lots of regular touchups/reapplication & is tacky/messy)
I really recommend you try Amsoil's HDMP - particularly this newer formula they came out w/ fairly recently. I WENT TO TOWN on my Raptor w/ it, dropped the spare scuffed any already rusty/oxidized bits up w/ scour pad, cleaned/wiped everything down & made sure it was bone dry before applying. I painted the rear axle/diff w/ rustoleum first. I think I used a total of 5 cans on my truck & before winter I'll have my mechanic use another 2 cans to really have a semi-permanent protective coating. You can literally see the extra layer of "soft texture" to the frame/undercarriage, but it somehow is basically dry to the touch & not only hydrophobic it also repels dirt/dust somehow - that is the biggest difference between fluid film + the consistency, it goes on runny but literally dries to almost a bees wax consistency, not greasy film.
I backed my new waverunner trailer in/out of a salt water launch this past week a 1/2 dozen times - the back section of the frame got a good soaking in salt water, all i did was hose off lightly when I got back home, the waverunner, the trailer, & the sides of the bed still had "salt dots" or water spots that required additional cleaning/rinsing etc. to really get the salt completely off, while the frame was spotless & still had the slight tacky texture of the wax coat fully intact.
Side-note - I used this stuff called "Yamashield" a metal protective spray for marine applications by Yamaha, & I think it would also be a great product to use on any bare AL parts you'd want to coat or anything that you wouldn't want fluid film gunk or even the HDMP that will tint some stuff yellow once applied & is more visible. This Yamashield stuff is probably just as heavy duty & dries to a clear coat, it is safe & even recommended to use on plastic & rubber parts.