electricity questions?

Ok along the same lines as what Prodart340 said, if it were me, here is how I would do it. As long as you have a 1" or larger conduit between your panel and the garage I would do this:

Install a 2-pole 50 amp breaker in the main panel, pull two black #6 AWG, one white #6AWG, and a green #6 (this could be #8 if you wanted it to be) from the main out to the shop. Use a good wire with XHHW insulation and use copper, yes it costs a little more but it is worth it in my opinion. Set a smaller sub panel in the garage and split off the 220/30A for the air and put a dedicated 120/20 amp circuit for the welder as well as another 120/20 amp general purpose outlet circuit. If your garage is a metal building it would be an excellent idea to bond the steel from the building to the ground bar in the new sub panel.

Another tip that will help is to keep your cord to your welder as short as possible.

Jason