Flux core wire is too large. The smallest wire you can get in flux core is .03. .23 is much better for a beginner welder. Use 18g steel for patchwork. You can cut to fit and start your arc on the 18 with takes the initial bump of heat. Then wiggle your whip and let it flow onto the thinner metal.

If I had it to do over I'd buy a huge bottle of CO2/Argon and not fool around with flux core. Flux core is great for heavier metal, especially outside with the wind blowing.

If you're working on structural or the skin of the body you might want to find original panels which are rust free and always cut more from the donor than you can possibly use.

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84 300SD
85 380SE
83 528e
95 318ic