.024 is good for doing sheet metal. The .035 is for heavier gauge metal like roll bar tubing. The welder should tell you what heat range and wire speed settings to use for each gauge of steel you want to weld. Lots of practice is the key to good welds, and in different positions - sideways, straight down, straight up, like the different positions you need to be in when welding a cage. Well fitting pieces are the other key to good welding - tight to each other and cleaned of paint, rust, etc. Don't go anywhere near the car til you feel confident in your abilities.