Wow i never knew you could get arch rods that weld ali. what rods are they? link? are you sure the ali is clean enough? try to sand it first as it starts to oxidize pretty much as soon as it hits the air. i have no experience welding ali because have no access to tig but that info i just gave you was off of a program i saw on discovery channel :P

EDIT: and if you smear washing up liquid on the backside (haha) of your weld it starts to go a funny colour just before you burn through. so you know you've penetrated (snigger) as far as possible without burning a hole.

Hope that helped.