[Kevin in NJ]

I suggest that you go to smartflix.com and get a couple of videos. Fender Arches and Shrinking Metal (or something like that) are the two that stand out in my memory.

Mig welding has some drawbacks that you need to learn to work around. The welds tend to be harder so when you planish the weld to correct shrinkage it acts strange. This is worse when you have an uneveness to the 'lump' in the middle. You will understand as you try to work the metal. The problem is this can introduce some distortion. Not much you can do about it.

Making sure you have full penetration is first. When you look at the backside of the weld it must be melted together there too.

Slight gaps in the metal or grinding a taper on the edge of the metal may help the weld smoothness. I have not practice this as I learned them after I switched to TIG.

Copper. It is your friend, put it behind the weld. It help control the heat. Not to prevent shrinkage!! It helps control the heat to prevent burnback of the edges. I even used copper wire inside the loop at the bottom of the hood sides so they stayed hollow.

I also highly recommend you buy some ER70-S6 (or what ever type of wire you are using in the MIG) TIG rod. When you are trying to fix small holes or burn back start the arc on the rod in the hole and keep feeding the wire into the puddle. You will like how that works. This also is a way to fix some deep pits that may be away from an area that you are replacing.

I made my own patch panels up from scratch. See the link to my website below. You want to use new 20 gauge metal if you go that route. As you make the panel you must think about how pulling the metal will affect the other areas of metal. You can do some complex stuff with a little bit on knowledge. Kind of fun.