the HEAT on a MIG is slightly more consentrated and generallt the best method is to PUSH the weld rather than PULLING it as if you PULL it it will build up a PILE of weld which will simply fill up on TOP rather than heating enough to penetrate well. by pushing the weld it does pre-heat the ateras more BUT it also depends on HOW you can get into the spot, corners over heads ect will determine how you weld in many cases, but flat long butt welds need to be pushed at an angle of about 80 degrees not quite straight up but tilted enough so you can see the weld puddle and the seam. same for fillet welding (inside corners) outside corners require less heat and slightly more travel speed depending on thickness.
I've welded everything from 22 gauge stainless steel to milti-inch thick steel & alum bars. using MIG, Stick, Heliarch/TIG & Spray arc on them. always wanted to try some of the carbon arc stuff but hey I kind of gave up working and became an electrician /forums/images/graemlins/wink.gif
mark M