to check for rust developping under the plastic ground effects (skirts) remove the two rear pannels (from the back fenders). If you find rust spots smaller than say 3" diameter, probably you don't have rust under the other pannels.
Treat those rust spots with POR15.
treat the rest of the areas with some sort of liquid anti-rust. You don't need to remove the other pannels because probably you have very little rust there.
We are lucky, because those rear pannels are the easiest to remove.
I would gadly help with details on removing those pannels.
Good luck!