"Fluid Film" is the way to go. The navy has used it for years. I buy a couple of gallons a Year at NAPA, spray it on the Vehicles, no rust issues.
Edit to add, the underside only!
I've used this and also stole a recipe from someone else.
Mineral Spirits, canning wax, and a touch of detergent free oil. Warmed up and you can spray it through a normal sprayer. Works great.