this video gives a few tips for tig welding thin aluminum lap joints.
The main tip is to make sure to tack weld it up without any gaps.
things go bad quickly on thin aluminum lap joints with gaps.
So, however many clamps it takes or tack welds it takes to not have gaps, thats how many you should use.
I try to make my tack welds as small as possible...or at least smaller than the final weld but since aluminum tack welds tend to crack, sometimes you just have to live with bigger tack welds than you would prefer.
Ideally, a 1/16" electrode might be used for this lap joint but I dont like switching out electrodes often unless I really need to...so I just used a 3/32 2% lanthanated electrode with a tapered point.
( in fact, I use a 3/32" 2% lanthanated electrode for about 99% of my tig welding)
the tapered point allows for a crisp start without a lot of arc wandering and works good on thin metal with low amp starts.
If I need to weld up around 200 amps on thicker aluminum, I will simple round the tip of the tungsten using the AC balance knob set to max cleaning.
I didnt mention AC frequency settings but I did use a higher than normal setting of around 200hz for this thin aluminum lap joint.
Using a higher ac frequency focusses the arc a bit and helps to keep the bead from getting too wide.
I used a #6 gas lens cup for this but I have also used a standard #5 collet body cup with good results.
I used the CK worldwide mt200 for this video and I mention that because all tig welders are a bit different so my setting for AC balance might be different than other machines.
But just in case it helps, here are my settings: