I found the hardest part of aluminum welding was to keep everything CLEAN,CLEAN,CLEAN.
I clean the weld area with MEK or acetone to remove any oils and finger prints,then use a stainless steel brush or stainless steel wool to remove oxidation JUST BEFORE welding and then use the stainless steel wool to clean the filler rod to remove any oxidation from the rod. Aluminum can oxidize in MINUTES so clean just before welding. Most sanding disks and grinder wheels have resins that bond the abrasive together and will contaminate the weld area so avoid grinding the area clean. Carbon steel wire brushes and steel wool will contaminate the weld also.I always break off the contaminated tungsten but one of my teachers told me grinding the tungsten was OK! GO figure?
I use a 30 year old miller 250E-HF with no special controls like the new electronic ones have and once get it set to the amperage I want and everything CLEANED, welding Aluminum isn't really any harder than steel .
And remember the old welder's trick to pretty welds: go over any ugly welds with the torch and run a" line of fusion" to smoothen out the weld area to make it more appealing looking