The idea of putting an air hose directly on a MIG gun is really absurd to me,first off..MIG uses gas(argon or CO2),fluxcore is gasless and rarely anyone will use fluxcore for thin sheetmetal as in body panels.
Blowing air directly on the weld will blow away the gas needed for the solid wire MIG uses.
Another thing i see alot of folks say,trying to cool the weld quickly by a wet rag or blowing air on it,a weld will pull more by doing so,causing more distortion in the sheetmetal,the only way to get the least distortion is to tack here and there FAR away from each other and letting it all cool very often on its own.
We use the fast cooling method to remove dents in steel or chemical tanks with dents,heat just the dented area up,throw a wet rag or direct water onto it,causing it to pull/shrink up where the dent or bent peice is.
i would NOT recommend cooling a tack weld in this fashion on light sheetmetal you are tryin to keep from distorting as much as possible,it will only make things worse.
Ive welded several patch panels in vehicles and very little distortion but only by having pateints and only tack welding here and there a few times far from each other and sitting back waiting for it all to cool totaly,and then continue a few more.
Where most people make mistakes in ANY type of welding is they get in a hurry.
Relax,take your time and pay attention to the details and the end results will be rewarding.