Yea they are good on the thicker stuff but if you are really wanting to weld more than just an aluminum trailer-- a TIG is the way to go. I have worked with a spool gun before and I cant imagine doing it again and actually wanting someone to see the weld without it being all sanded. If you are building say a tank out of heavy material it will probably be a good option for you.
Just my thoughts.
I own a machine and fabrication shop that does a lot of aluminum fabrication. In that environment, the ratio between what we MIG and what we TIG is probably 80%-20%..maybe even 90%-10% MIG is FAR more productive; that is why aluminum structures like ships are 100% MIG welded (with some friction stir-welding thrown in perhaps). That said, we do TIG weld all of the fuel and hydraulic oil tanks that we build.
As previously noted, metal cleanliness and temperature are very important. The weld zone must be very carefully prepped and kept clean; only stainless steel wire brushes and a weld prep acid should be used just prior to attemtpting the bead.
We use 5356 wire almost exclusively. 4043 is a lower strength alloy and less resistant to corrosion. It's used primarily in areas where bead appearance is more important than weld strength.
We use a number of machines. Have two Profax spool guns, one on an Esab 250 and the other on a mobile Hobart Champion 16. Then we have an AIRCO wire feeder with push-pull gun that we feed with Miller CV power source..it holds a full size spool of wire. We only load that up for really big jobs...
Had another big ESAB 400A power source and full-size spool wire feeder but it got sent to some woodworking clown in Florida who said he could use it..
