My short 2 cents (I could go on for pages). A photo of what you're doing would be nice.
I would definitely avoid using a flange and plug-welding the holes. It's not recommend because it's a moisture trap and lead to premature rusting. I bought a panel flanger in the beginning but have never used it. Butt welding is the recommended procedure but it takes a LOT of practice and patience. When welding overlapping sheet metal, you should treat the mating surfaces with weld-through primer. I like U-Pol's copper based primer. Heat from welding doesn't burn it off. Not a perfect solution but better than doing nothing.
Too much heat (voltage), holding the trigger too long, having too much of a gap or not having the gun at the right angle can blow a hole in the metal before you know it. Too little heat and you won't get enough penetration. Heat from welding can warp the sn@t out of a panel in the blink of an eye. Heat from grinding and sanding can also damage panels. If you mess up a weld dot, you can't just add more weld metal to it to try and fix it. You need to stop and grind/sand it down and redo it before moving to the next weld dot. Sometimes you can get tiny pinholes in your welding and you need to check for that (worklight behind pieces works well) and need to go back and weld them up as needed and also, sometimes when you sand/grind the welding, you can find spots where you have imperfections that need more weld on them.
Beware - there a plenty of youtube vids by folks that have no clue how to properly weld sheet thin metal (like there is on Fiats). Tig welding is good for thin sheet metal but a tig welder costs a fair bit more and has a much longer learning curve.
If you were to use a flange, how would you hold the pieces together? Cleco fasteners would be an option. Sometimes bodywork magnets will work. If you don't hold the pieces firmly together, it'll become a mess. If you butt weld the metal, you can use welding butt clamps like in the photo below of work on my 600 body where I was installing new rockers. You can see where I plug welded along the bottom of the rocker because there are 3 layers of metal and spot-welding isn't an option. When butt welding, it's important to have an even and correct gap along the entire seam. This can be a challenge when fitting up large replacement panels with lots of shape and corners. You need to avoid sharp corners in the pieces and want a rounded turn in direction. Must check the adjacent pieces after each weld dot for accurate alignment as you do weld dots along a seam. Each weld dot should be planished (hit with a hammer & dolly) to reverse the shrinking that occurs in the HAZ (heat affected zone) or if you don't you can make a total mess of the panels. It's not recommended, but on thin metal you may need to use a wet rag to cool the metal immediately after each weld dot. Some use a blow gun. Below is a photo showing what happens to thin metal when warpage from heat wasn't addressed. Sometimes you can run into dirty/contaminated metal that can ruin the welding and sometimes even the metallurgy of the original metal can be a factor.
Do you have a mig welder, have you welded any sheet metal with it and how much? If no experience, don't go out and buy a mig welder and watch a few youtube vids and think you'll do just fine. The heat setting will depend on your actual mig machine, the gauge you are welding and a few other factors. I could tell you what I set my voltage and speed at but will mean nothing for your situation. A cheap mig welder isn't worth buying and can cause problems. You want to mig weld with an Ar/CO2 gas mixture. Old school restorers use gas welding and "hammer weld" the butt welding as they go along by using a hammer & dolly while the metal is still red hot but is pretty tough to do along and it takes a lot of skill.
Besides a mig welder, you'll need an air compressor with adequate cfm and an angle die grinder. Then an assortment of different grits of roloc sanding discs. A thin 3" cutoff wheel works well for knocking down welds before sanding the welds down. I usually use 2" and 3" roloc discs but for larger and more flattish areas, you want a larger sanding surface like a DA sander in order to avoid make sure the smaller discs don't dig into the surface. Flap discs and flap wheels can be useful in some spots.
If you can't buy replacement panels, sometimes you need to take flat sheet metal and form it. Another skill/art to learn.
Ideally, you want to aim for a good result after sanding down the welds so that 1/16" or less of body filler is needed. You don't want to leave bare metal exposed and you need to get primer on it pdq. Epoxy primer is great stuff. Body filler can go over the primer and some filler works well on bare metal (like All-Metal brand). After you've got primer on, there is a window of about 2-7 days to get the topcoat of paint on for a chemical bond. Otherwise you need to scuff the primer up. You can "smooth up" your welding with the All-metal on bare metal and then prime. After that you may find you need to fine tune the initial filler work with some body putty over the primer. Finishing an area after welding to prep for painting is a whole other set of tools and know-how. You may need to get into learning how to use a guide coat, sanding blocks, etc.
When butt welding a seam, you start by doing weld dots about an inch apart and planish them as you go along. If the weld dot is too high, you need to grind/sand them down a bit. Then you go back and do more weld dots in between the first round of weld dots. When you get to the point of filling in the seam between the dots, you overlap weld dots so you completely fill in theBelow is a photo of modification I did to the rear fenders on my 600 body in order to widen the opening for tire clearance. I held the pieces together with butt clamps and took my time doing the welding.
If you aren't planning on doing a large restoration project now or in the future and just have the one area to deal with, you might consider just preparing the car and fitting up the replacement panel and then hiring someone to do the welding then take it over from there. If you could get it tig welded, all the better. I have a partial new rear quarter panel to weld up on my Abarth 1300/124 project and may do just that. There is part of the panel that I can't behind to hold a dolly for planishing and can't risk the welding turning out badly.
If you do decide to do your own welding, do lots and lots of practicing on scrap pieces of thin sheet (22 ga. and thinner) until you've got the hang of it. BTW, mig welding on thicker sheet metal like 12-16 gauge is MUCH easier and when you get into 1/8" and thicker, any monkey can do it... Don't forget to post some photos on how it turns out.
For some excellent how-to photos and advice, look up forum member "tonybmw" on the retrorides.com forum in England. Best info. I've found on the internet and he uses some pretty ordinary and low cost tools. He has one particular long thread that is fantastic (work on a BMW).
View attachment 19087 View attachment 19088 View attachment 19089
