Welding is the best way to go if you can swing it. Alumalloy I think it's called, is another good way to plug holes. It only needs a propane or MAP gas torch, and works pretty much the same way solder does. Factory seams and holes were filled with lead. This is basically the same thing. Just hit your holes with a wire wheel first so it sticks. The aluminum can be sanded down with a regular DA sander. Steel welds need a bit more aggressive grinding type wheel.