I don't bother with pre-mixed gases- - - -I have a bottle of CO2 and a bottle of Argon, T'd together upstream of the welder, and I adjust the regulator pressure to create my own mixes for mild steel and stainless. I haven't tried aluminum yet, but the tip about using stiffer wire sounds promising. My mix for stainless is about 90% Argon and 10% CO2. The weld is a little dark, but a quick buff with a stainless wire brush in a die grinder slicks it right up.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!