Can't say more than practice, practice, practice. First time you get near the car, it's always difficult, as invariably it's a bit thin, upside down, back handed, can't see it etc etc! The MIG machine makers will supply a hand held mask, making you believe that you can weld one handed. Ignore this, and buy a proper flip mask - MIG is much, much easier two handed!The basics is to set the power to make sure you are melting both sides of the weld, then set the correct wire speed so that the wire is melting at the right rate for the seam you are trying to lay down. When you get it right it settles down to a fast sizzle - often likened to bacon frying. Too much or two little wire and it will spit and pop. Too little power, and the weld won't penetrate, too much and it will burn through. Move too slow and it will burn through, too fast and it will sit on on top and not penetrate. Torch angle is critical as well - it needs to be at right angles to the weld (which usually means cranking your hand round, as the torch is really the wrong shape). And you need to get the right angle to the direction of the weld - ideally it should be right overhead, but do this and you can't see it, so you have to lay it back a bit - not too much or you'll lose the gas shield though. Distance of the torch from the work piece is also vital - too close and you just block the tip up and weld the shroud to the job, too far away and the wire wanders about and you lose the gas shroud. Still sound easy?? At least setting the gas flow is easy! By the time you've got all the variables right, you've got to the end of the job usually!
I have to disagree with BXbodger though - the moving fast is the wrong technique really, and is likely to lead to a surface weld with little penetration. You need to get enough power on the MIG to melt right through the thicker of the two sections you are trying to weld - and this usually means burning a hole if you try and weld the whole seam in one go. It's better to pulse the MIG (expensive machines will have a pulse timer built in for this) - you weld for just long enough to get the penetration, but before you burn through, then stop for a short while, then weld another short burst. If you watch the weld pool, you can see it just 'skin' over as it cools. If you re-start welding into this, you end up with a weld that looks as if it was laid down in one go, but gets good penetration without burning through. Trouble with cars is that usually you can't see the back, so can't tell if you've penetrated or not....
The reason that my welding of the Visa stalled is that I got into a difficult corner inside, and couldn't get it right, so got fed up. Need to get an auto dimming mask really - trouble is installing new kitchens and bathrooms got in the way this year. Next year. Next year!
Whereabouts are you Ralph? Maybe someone can give you a go with their machine so you can see what you are up against.
D'uh, Bradford I see....not near me then!
](./images/smilies/eusa_wall.gif "Brick wall"){kind=small}
! Its always hard to see what you are doing as you're usually lying awkwardly in the gutter on your back or side, and the torch movement tends to slow down(...and who hasn't then tried to fill the resulting holes with more weld..... 
....it never works! )
