Was flicking tv channels yesterday and caught an old episode of The Repair Shop.
Guy was fixing dents in the tube of a vintage telescope with a ball bearing probably in excess of an inch diameter on the inside and a large magnet on the outside.
He said the magnet had a pull of 26kg and he slowly massaged the dents out by working the ball over the dent with the magnet.
Wonder if this would work on fuel tanks, exhausts etc. I appreciate the metal is probably thicker but might be worth a go.