Or try stickwelding it with 6011 rods, it'll look like sh*t, but this stuff goes trough anything, rust, paint, undercoating... you name it.
It's usually used to do the first penetration pass when welding really thick and not so clean metal, then after being cleaned with a good wire brush, the rest is filled using 6018 or 7018. Since you're welding sheet metal, you don't need to cover it if you don't mind having welds that looks like they were done by a beginner.