Well, in '83 the metals used in vehicles isn't the same as it is now....plus now new vehicles are about 30% plastics and plastic doesn't rust.
Yes, rust proofing IS a scam...unless you're going to keep your car for 25 + yrs. Most manufacturers have a 10 yrs + rust warranty.
If it is a used car, with no warranty, and wasn't rust proofed at new, doing it now really won't accomplish much. The damage has already started and if you do it now, you could just lock in any corrision beneath the undercoating and help it along.