I do mine after a big rain and drive the truck through the water puddles so you can hear it beating up under the floor pan the go to the car wash and get the under car wash. Put fan under it for 24 hours the go at it.......jk

Does it work over rust?

it blocks the air getting to it so it slows it down more than doing nothing

I put mine over rust. Trying to slow down the process.

I use it on our quonset building where the metal meets the concrete trough. It begins to rust there and below grade.