Even with a c clamp or other clamping or magnet, the easiest and cheapest is still hit it with a 4 or 41/2" flap or grinder disk to remove the rust. The other thing you have to consider is that each piece of steel you add will need a mating part cleaned for good contact. Its just a fact of life. Clean you clamp and the steel.
Glen
Been there, probably broke it, doing that!
I am not a lawyer, and never played one on TV!
All the usual and standard disclaimers apply. Do not try this at home, use only as directed, No warranties express or implied, for the intended use or the suggested uses, Wear safety glasses, closed course, professionals only