I was new to MIG welding when I took up this project and have learned a lot by watching Internet videos, DVDs, reading and practice welding. Here's a short list of things I have sorted out:
1. Calibrate your MIG welder wire speed by timing how much wire comes out in 6 seconds (and then 10x for the per minute rate) for each major increment.
2. Set your amperage at about 1amp per 0.001" thickness: 40amps for 20 gauge metal
3. Use 0.023" wire and run about 3.5" per minute wire speed per amp: 120" per minute for 20 gauge metal
4. Practice and fine tune your amps and wire speed: look for enough amps to get decent penetration but not so much that you easily burn through the sheet metal. Get that nice bacon frying sound by playing with your wire speed.
5. Use copper backing when-ever possible when butt welding
6. Use overlapping sheets of metal when you can get away with doing so
7. When you have to butt weld, spend a lot of time getting the edges to touch everywhere and file the edges so that they aren't too rough as otherwise the edges burn back and set up conditions for burn-through
8. Putting heat into sheet metal causes it to warp. When liquid metal cools it also contracts and will pull the metal. Therefore try and wick away the heat with copper backing and/or use compressed air after 1/2" of welding to cool the panel.
9. Stitch weld when you are at risk of burn-through. Try 1 second on/1 second off. Essentially you are looking to start the next pulse just as the metal cools enough to not be bright red. Be aware that pulse welding may not put enough heat into the metal to give quality penetration.
10. If you are grinding the weld flush after welding be aware that you risk weakening the joint if you grind completely flush. Try grinding it back and leaving a small bump and then using body filler to smooth it out - this give the appearance you are looking for without the risk of a weak joint from too much grinding.
11. When you are welding vertically you will need to turn up the wire speed somewhat in order to keep the welding quality up. Increasing the speed will increase the amps (by lowering the resistance) and the additional weld material will help you get the vertical welding quality you need. Welding overhead is a bitch - you'll get molten metal all over yourself - be aware that you can really injure yourself so avoid this or minimize this.
12. When you get into position practice the weld motion before actually completing the weld. You may have to adjust you position.
13. Rest one hand on the metal to to welded and support the other hand/gun. Get your head/eyes within 6-8" of the weld. It is easier to see what you are doing by pushing the weld, rather than pulling it. Have your gun tip fairly close to the material, as in within 1/2". Keep within 20 degrees of vertical position with the gun tip to the metal. Cut the wire to about 1/4" sticking out of the gun each time you weld. You will get cleaner 'starts' after a clean cut.
14. When turning up the amps to weld thicker material, you might have to turn up the wire speed more than you think: if you get a 'pop' and the wire has burned all the way back you know your wire speed is too low.
15. You can play with the weld temperature by adjusting the gun tip distance to the metal surface. As you pull back you are increasing the resistance and lowering the amps (and the heat). If you need to fill in small holes or burn-through mistakes you can try pulse welding from up to 1" away at a 45 degree angle to the edge of the hole. What you are trying to do is deposit a small amount of weld, let it cool, and then deposit some more. This often makes for a cold/weak weld but then you can go over the area with more weld/heat and fuse it together better as a final pass after the hole has been filled. It doesn't always work out well but often does. Having a copper backing plate will make filling holes a lot easier and more successful.
16. Double check all of your welder settings and make sure that the wire is feeding correctly and you have proper shielding gas flow when things start to get nasty. I have found that the settings have sometimes been accidentally changed by bumping into them or equipment is not working correctly.
17. Clean your weld surfaces prior to welding and use pre-paint prep to remove any oils/contaminants. Any paint will screw things up too. Weld-thru primer is ugly. If you have to use it, stick to the back side of the metal and grind if off where you are actually trying to weld. Seriously.
18. Make double sure that you have a solid ground to the metal.
19. Always start your weld on the thicker material, establish a quality weld pool and then move to the other metal surface. When you have materials of equal thickness, start on the metal with the better ground and move to the other surface.
20. Use a bright light that can be focused on the area to be welded. It really helps. Weld outdoors in the sunlight if possible! Use an auto darkening helmet. If you use reading glasses to read use them when welding too. Good visibility is critical to welding quality/success.
21. Clean your MIG gun tip every so often and use the MIG anti-splatter grease on all parts of your copper tip - keep it looking spotless, inside and out.
That's the majority of it. Be calm, don't forget to breath while you are welding, and if you start screwing things up, take a break, calm down, check your complete setup, and if you still are having problems, go back to welding on some scrap material to see if you are doing something wrong ... it is usually unclean metal, the wrong settings or a bad ground ...