Re: new skeg? mig or tig
Depending on what size motor. Whether or not the lower unit is attached or not is not that big of a deal, depends on the welder's preference. It's much easier to weld if the lower unit is removed. The problem is welding heat. On the larger motors 85HP and above, the casting is pretty substantial and as a result you may end up putting a lot of heat into the weld. The lower unit gets pretty hot and you may end up damaging the seals in the larger motors. The spool seals are pretty close to the skeg. Safest way is to pull the spool assy out with the seals (replace seals if necessary) and TIG weld it. That's the way I do it and have not had any problems. I'm sure there are other ways to do it, but this has worked well for me.
Never tried to MIG weld aluminum since I have a TIG welder. I've heard of some folks using the Alumaweld product with a blow torch but I've never tried it. All of those products like the Alumaweld are basically a zinc alloy with a melting point of about 750? F.