Hi Mike, welding of the outer fender should be reasonably straight forward, as long as where you are trying to put the patch in, is tying in with good sound metal. It has to be super clean both inside and out as when you weld it tends to draw any impurities from the backside through. If you have a mig and wish to try it you have to use pure argon as sheilding gas(as stated) buying wire shouldn't be to bad .035" (.9mm) 4043 is available in small spools (I've got some if you wish to try). Make sure that you keep the gun lead as straight as possible, give the liner a good blow out(generally best if you take the liner out of the cable and blow compressed air through whilst trying to rap it lightly on the ground). I would generally go for the next size up tip to help the wire to feed freely (all helps to avoid birdsnests when you get burnback). Probably another thing to check is that the feed rolls line up with each other in the vee, if they are offset a little they tend to shave the wire which then builds up in the liner and then makes nice birdsnests. The feed roll pressure should only be enough to feed the wire smoothly(my simple test is to pinch the wire between your thumb and finger and adjust feed roll pressure until you can't stop the wire with reasonable pressure) When you buy material to make patch ask for 5052 grade if available it is a marine grade 5% magnesium, it is a little stronger than 5005 so it will help hold it's shape a little better. Not sure where in Vic you are but if your sorta close to s/e burbs I could do it with the Tig (dynasty 200). please feel free to sing out if you have any questions or want any help just pm me, or from anyone for that matter
regards