I have a gasless mig which i use for panel repair. Fibreglass works but only if you have a clean, dry and somewhat coarse surface for it to bond to. I would tack a 1mm plate behind the area (which would be very difficult to get to) and then fill the holes with weld or metal and grind and weld and so on till you get the disired finish. The whole area would need to come back to metal.
Other than that you can cut out exactly the area (using a donor metal) you have to fill and then slowly weld that in place. It is very easy to blow holes in sheet metal even with a Mig.