I have had Rust Check applied to two vehicles. The shop I use does a good job spraying inside frame rails, door jams,,, They will drill holes in door jams to be able to spray inside areas, so be aware of that. They plug the holes with plastic plugs.
I think that it works, but it is not a one spray and your done program. They want you back every year. I get a re spray every two or three years. The price is a bit less with the re spray.
The car will drip for a couple of days, for me that is fine, I park in the street and keep it out of the garage. No big deal, it is surprising how long the drip marks lasts on the street surface, to me that is a good sign.
The under side of the car dries eventually, no big deal to work on the car .The oil coating likely lasts longer in tight crevices and places where water would collect and start rust.
With that said, if there is already rust started, an oil film may slow it down, but it won’t stop or fix it.