A spool gun for Aluminum is great for aluminum sheet work where you need to utilize speed. I recently sold my Spool gun for my Miller that i used to Fab cabins on boats in the late 80's early 90's. It made quick work of short stich welds and inside corners on .100 & .125 sheet. But on the outside rubrail we used our old Heliarc machine to get a nice looking clean strong weld. Any fuel or oil tanks we built were done with the old Heliarc machine also as you can produce a better seal on the outside corners. As mentioned in another post here Tig will also give you the ability to add some cleaning for the old dirty and oily parts.