I had to cut a section of my cowl out in order to push out the firewall where the front fender attaches to the firewall.

I got the piece welded back in, but I have a lot wire build up and spots where the metal burned through. I was trying to fill a 1/16 gap with the MIG and in some spots worked great but others I got burn through.

Also the back side of the panel I welded has pieces of the Mig wire sticking out, is this going to be a problem with corrosion?

I'm using .25 wire with a 220v settings are voltage at 5 and wire speed about 3.5-4.0.

I once did a floor patch and it looked OK from the top and there was build up is spots and pretty ugly looking mess. I covered it all up with under coating stuff to seal the floors and firewall.

When I got under the car to do something else one day that patch looked like a freakin porcupine on the bottom with wires hanging out everywhere. LOL

I ended up grinding it down on the bottom (watch out those small pieces of wire break off when the grinder hits them and act like shrapnel). Once smooth I welded it again from the bottom and of course burned off the sealer I had put on top LOL, so I rewelded the top again as well. Practice makes less than perfect LOL

There are several things you have to get right to avoid that problem, wire speed is critical as well as voltage and amp settings. You need to get some old sheet metal and practice until you find a setting that works well on the metal you are welding then write it down somewhere so when you start up again weeks later you have a good base to start from.

I am about as far as you can get from calling myself a welder and it's trial and error for me everytime. Someone here will likely provide more technical data and I just wanted you to know you are not alone in the porcupine department LOL