I found with my Lincoln 200 tig I could weld aluminum ok one day, not so well the next. If I hadn’t welded aluminum for some months I had to practice for a bit before attempting the job. It wasn’t that way with steel because I learned to weld sheet metal using oxyacetylene, back in the 80s. Not especially well. The learning curve is longer with aluminum. Everything really clean. Tight gaps. Flame / filler rod coordination is critical as are your amperage and AC balance/frequency for the filler rod you’re using and the thickness of the aluminum alloy you’re welding. What electrode are you using? I like 2% ceriated, blunted tip. What’s your scfh setting at? Are you using a regulator or a meter? I’m using 15 scfh through a meter, mostly because it’s more efficient delivering the Argon consistently.
Watch a bunch of videos. Practice longer.
I’m sure not great at aluminum. Passable. This advice is not coming from an expert. For that watch those Youtube videos.
A good machine is essential. I have an R-tech analogue machine which works well but wasn’t cheap - obviously must be AC for aluminium, mine is switchable. Butt-welding 1mm sheet aluminium is perfectly do-able but takes some practice. Gas should be pure Argon, without filler rod. The two edges must be clamped really tight together, and preferably in free air, not on anything that will act as a heatsink. Aluminium conducts heat so well, you need to do everything you can to keep the heat in the actual weld. I use a finger switch rather than foot control - no filler rod - by the time the rod has melted, you will have blown a hole in the joint. Tacking is the real art. A quick burst with the torch about 15mm from the joint on both sides in quick succession - this expands the metal slightly to force the edges together a little more tightly. Then immediately start the torch on one side, run up to the joint, along it, then off it to the other side (think an ‘S’ shape movement) and release the switch. Current should be 70-80A, which sounds high - but you have to hit it hard and fast or the metal will melt.
Running in the weld is comparatively easy as the joint should be nice and tight so won’t blow into holes. Drop the current back down for this, maybe 40A. If you’re working off the car and have a wheeling machine, give the joint a few runs through the wheel and the weld will be virtually invisible, no grinding etc. needed.