Having trouble with Tig welding

 Peter, my new welding buddy, had some strife getting Tig to work.  He hasn't done a lot of Tig, but we're doing a 70 year old project truck and the metal is thin, so he's going to have a go. We bought a welding gun (Michigan) and connected to my Unimig 165. His first few tests blew right through 3 mil plate. He went to test on 5 mil and it burnt through the other side. He said the setting was for 90 amp but he dropped down to 40 and still doing same thing. He said the tip should not be looking like this after only a few attempts. We gave up and went back to mig. Any thoughts? Wrote the story up on Welding Australia Facebook site and the responses were overwhelmingly in favour of a couple of key issues:

1. Was the gun plugged into negative port?

2. Was the gas flowing?

Some suggested Thoriated (red) tips, but these should have still worked fine. 

This is the back of a sheet of gal. Peter had the setting dialed in where he felt it should have been, but you can see how badly the weld went basically right through the metal. This was a sheet of 3mm zinc. 

An example of the tip after just a couple of attempts. This should not be happening.

Peter says he had the leads in the correct order and had the gas working fine. He even tried welding some 5mm plate and incredibly, the weld even showed through. We tried a piece of scrap rusty steel and it obliterated it. 

So, after an epic fail with the Tig, Peter went back to the Mig.  I am hoping he will have another crack at things this weekend, as there is plenty of light work needed on the cabin.