Fluid Film (Sheep Wool Grease....ie Lanolin) will stop it from Rusting More.
Astro Sprayer is $15 on
Amazon - You need to supply Compressor.
Fluid Film is $43 per gallon and I just used 2 gallons Bathing my Chassis in it - Ohio Truck.
Drop the Rear Tire and spray all up in there and check condition of Spare - Mine is Trashed.
Wear Glasses and hit everything with a Wire Brush then Spray away.
Wrap Exhaust in Newspaper or let it burn off.
Make sure all mechanical work is complete before Spraying else it will be all over you and clothes.
Wear Tyvek suit if you have or Junk clothes.
*If you do not want to spring for Fluid Film Chainsaw Bar &Chain oil is $13 per gallon and China*Mart. It will work as well.
Grease/Oil creates an Air Barrier which stops Rust, but Flaky Rust must be Removed 1st else it cannot reach the metal.
**I Buy 5 gallon bucket as I have 5 Old Toyotas.