Not "any type of undercoating" will trap moisture.You could clean and paint with a self etching primer. Be careful with any kind of undercoating around salt water that could trap moisture in. When I go down to the beach in Mexico I like to spray the underside of my vehicle with Fluid Film first.
Isn't that what I said?Not "any type of undercoating" will trap moisture.
My suggestion to avoid rust is to use a lanolin-based spray like woolwax, fluid film, surface shield, etc. They're simple to apply, albiet messy, and need to be touched up or reapplied annually, but they'll arrest existing rust, displace moisture, and work into crevices and such. And they're cheap too, especially if you have an air compressor and apply it yourself.
A sprayer seems really smart.Isn't that what I said?An easy way to apply Fluid Film is with a pump up weed sprayer. I do my Jeep once a year before driving on the beach.