[Aussie Mike]

That AC balance I've found is an important setting. It controls the offset between negative and positive in the AC current flow. The positive current flow is what does the cleaning action. It blows the oxide off the surface of the material. The negative current flow provides the heat for weld penetration.

It's also sometimes called "Clean Width" on the dial. You can experiment with it on some scrap material running beads back and forth and see what difference it makes. The more positive you set it you will see the bands at the sides of the weld widen as more of the aluminium is "cleaned". You'll want to set it so you get a nice neat weld with not too much discolouration at the sides from the cleaning action.

Edit* Another thing to remember/observe is the Positive current flow puts more heat into your electrode than the Negative. You will find your electrode will ball up quicker with more positive offset. With some trial and error you will a find a happy medium that works for you.

Cheers