2. Is your aluminum anodized? If so, then it will be harder for the arc to break through the coating, unless you sand it off.
3. Are you brushing the joints with a stainless steel wire brush, prior to trying to tack?
It appears that you are attempting to learn to tig aluminum as you fab this project. May I suggest that you practice on some scrap pieces of the same dimensions that you are working on? I just ran a bead on the 1/2" thick end of a piece of irrigation pipe with my 165 (preheating with the arc), so you should be able to do similarly with your 180.
The fact that you are getting soot and melting the material before getting a puddle indicates that you are not applying enough heat from the git-go. Get a few practice pieces and work on developing that puddle by flooring the pedal at startup. You can take advantage of some pre-heating by moving the arc around in the direction of where you want to go, then retreat and add filler to the puddle. It is all too common for a tyro to hold back on puddle initiation for fear of melting the work piece, but what you need to do is floor the pedal to get the puddle started and then add the filler, as you progress. Remember to go hot and go fast (relatively speaking). Aluminum tig is not for the fainthearted.

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