Phil,
Well, you may say the job was complete. Good idea on taking the skid plate off.
Do you have any idea what was applied to kill and prevent further rusting? Something that could be bought at an auto paint supply?
Since having only one small shop that does rust treatments, I may end up just keep after the rust with rust converter myself. I know that's a never ending PITA though.
The salt air is so thick around here everything that can rust, will rust and fast. I've seen vehicles parked outside at the beach, get serious rust damage within one year. It helps to do a daily rinsing and frequent thorough washings.
It shouldn't take long to determine how effective the treatment is. You're exposure to the salt air is very much like mine. Just a downside of living near the beach. Time will tell.
Money well spent.