Many years ago, used a chemical paint stripper designed to remove paint form aircraft purchased from an auto parts store. Many not be able to buy anything that strong any more. If I would do it now, would take it somewhere and have some sort of media blaster remove the paint.
If you want to see the original metal surfaces/ weld HAZ then you probably want to avoid media blasting and also avoid using anything besides white scotchbrite to loosen the coating. (yes, this will be frustrating) Also might be a good idea to throw down some good 2-part clearcoat once you get the frame cleaned off so the steel doesn’t rust too quickly.
Mar-Hyde Tal-Strip II Aircraft Coating Remover is the best stuff you’ll find for stripping paint from a kart frame if you can find it. I used to buy it from Kragen Auto Parts. May not be able to get it from an auto parts store these days… it is nasty stuff! Aircraft Spruce might be a source.
The aerosol can version is fantastic but be very careful to use in a well ventilated area with no breeze and wear gloves and clothing that cover all skin. I used the stuff to strip a Tony chassis and it lifted the factory coating off with ease.