Any time you weld using a MIG, you'll find that the area of the weld is harder than that of the parent metal. Weld two pieces together, grind the weld perfectly flat, and cut across the weld with a pair of tin snips. You'll be able to tell exactly where the weld is. If you planish each weld dot individually as I mentioned, working them becomes much easier. If you weld multiple weld dots next to and on each other, this tends to harden the welds even more, making any planishing efforts all that much harder and less effective in stretching the panel back out at the weld.
Things that you can do to overcome some of these issues:
Most welders are supplied with ER70S-6 wire. EZ Grind and ER70S-7 will both give a slightly softer weld, and lay slightly flatter, so they help to overcome some of the hardness issue. I'll add to that by saying stacking weld on top of weld with these wire types also seems to make the welds harder. Your best results with planishing Mig welds will be planishing them in the isolated fashion. Next, sometimes we all blow a hole and need to fill it. A trick that may help is to pull the trigger momentarily and then pull away from the panel. You should see a small weld glob or ball at the end of the wire, still glowing. What this will do is give you a wider "filler" to hopefully span the gap or better fill the hole. So while the ball is still glowing, do a real short zap on the gap you need to fill. Hopefully this will help out. The other thing with blowing holes, try bumping up the wire feed speed slightly. Next, if you do blow a hole, go back to your last weld dot, and overlap there. That dot will give you more mass to weld to and bring the weld out from there. Keep in mind, weld, planish, grind, repeat. Even when working like that to build out to fill in a blow hole void. The shrinking tends to pull even more when gap exists, so you'll need to planish in between each weld to stretch it back where it should be. And lastly, a lot of issues will be due to inconsistent gaps in panel fitment. You'll be much less likely to blow holes with a tight fitup than you will with a slight gap. (in addition to the benefits mentioned previously) When people discuss welding methods and setup, panel fitment for tight joints is absolutely part of that equation. You will experience best results by working toward consistency in every part of the process. If you can get to where it is almost OCD consistent, you'll see much better results.
Here is a thread to help show the weld, planish, repeat, and the results afterward. This is also using the -7 wire I mentioned, and you can see how the weld dots lay nice an flat. Where the -7 wire may be more expensive, it works out to less weld proud for easier planishing and for less grinding needed, which means less grinding and sanding consumables used, and less time for you to do so.
http://www.garagejournal.com/forum/showpost.php?p=2924553&postcount=53 Or read through the entire thread following the link in my signature
