My car had sat from 1974 til now.
I worked under my car- on axle stands- for 21 months.
I used torch and scraper,wire brush on my drill to clean up,and hammer and an old beat up flat head screwdriver to chip away the brittle stuff..
I found a lot of surface rust under my undercoating so I had to remove it..I cleaned up it to bare metal,a coat of expoxy primer and then painted. Some areas were pitted already and I treated them with phosphouric acid and/or a rust convertor.
Not perfect,but I know it will never see snow or salt and will last a long time.