Here in Vermont and over in New Hampshire in New England (US) for decades garages and car owners have been undercoating cars w/used motor oil yearly with great success. Road dirt builds up on it and forms a layer that is about 1/8" thick. With continued applications I have seen it be as thick as 3/8"
Done correctly and thoroughly and by also spraying it into cavities, and doors, and by plugging the drilled openings with rubber plugs, cars do not rust at all it the drains are kept open and if they do it is only minor and years down the road.
About 30 years ago there was a switch to using hydraulic oils as it will not rot rubber parts and seals. Today
"Fluid Film"is used, it is a lanolin-based product that is non toxic and will cause much less harm if it falls on roadways and other areas.
The average cost today is about $125 when done in a garage or specialized facility.
Learn more here.