i mainly focused on the frame rails, under body, and skid plates.
take the skid plates and check on the inside just in case. drop the spare tire and coat that and where the spare tire is. the main area of focus is the rear bumper. coat the inside on all three sections because that is were all the salt will be trapped underneath. i also did the rear axle. if you can wash the undercarriage at least once a week you should be ok. i go the car wash and try to angle the gun underneath the truck to get the salt off. you can spray paint the brake lines, fuel lines, and control arms but i wouldn't recommend using the rubberized undercoat on them. before you start the process, make sure the undercarriage is clean.