I had the bright idea of using a really strong magnet to hold 2 bits of steel sheet together while I tacked them. The magnet is super powerfull (used to rescue toold dropped overboard from boats etc. and has two "edges" so that it would straddle the joint lengthways. It all worked well except where I tried to tack where the magnet actually was. It seemed to really upset the weld, coughing and spluttering, the tack looked aerated and had no strength. Is it the magnet doing this?