We need to found and what measured thin metal is. There is no total "according to Hoyle" answer, but I forever considered something less than 3/16" to be thin. So from this point on we are talking regarding less than 3/16" material.
The first thing to shape out is what type of welding rods and what size of welding rods to use. If you contain nice new 1/8" material, I would go with 3/32" 7018 rods. If you include old rusty painted up 14 gauges, I might be tending to use 1/8" or 3/32" 6011 rods.
You may ask me, why don't you just crush the rust off? Well if it's just outside rust go ahead, but if it's pitted out you will crush all the material away before you get it clean. Not only have that, but if you are just doing a rapid repair job on incredible, just welded it. Probability is, if the material is that bad the weld will distant out last the rest of the material at any rate.