This was my first welding experience and I learned a lot from it. Before I attempted this I did get some scrap sheet metal and practiced until I felt comfortable with the welder. As you will see I butt welded the new panel in place to put the car as close to original as possible. As I found out butt welding 18 guage sheet metal is difficult because it is so thin its easy to melt a big hole right through it. Plus bending over and welding in the trunk of a car is a difficult position to work from. Despite these challenges I took my time and kept at it until it was done.
Then I used a sharpie and drew a line which represented the middle of where the panels over lapped. Next I cut along that line with a saws-all. Now there was no metal overlap in that section and a slight gap existed between the panels which is what you want when you butt weld.
Next I installed some butt welding clamps. These clamps align the two metal sections and hold them in place while its welded. Then I welded some tacks, one tack at a time, let metal cool, move to a different area, place another weld, let metal cool, and repeat. If you don't let the metal cool and move around you will warp the metal and blow big holes through it. You have to take your time with thin sheet metal!
As I progressed I removed the butt welding clamps. Then I repeated the above two steps for the remaining three sections.
After the panel is cut and tack welded into place I slowly added more tacks one at a time. I would add a tack, let cool, move to another area, add a tack, and repeat. Then once it was all welded up I used an angle grinder to grind the welds down. Typically I would find there would be holes and I would have to add a weld and grind again. It was best to turn the welder power down to prevent blowing a hole in the metal. I used a light to find the holes and got it fully welded all the way around. A very time consuming process. Once I got it welded and grinded I used a "flap disc" in an angle grinder to do the final smoothing.
Couple key points when it comes to grinding down the welds. First do not over grind and make the metal thin; you just want to grind the weld down even with the metal and no further. And do not grind continuously in one place. That will create excessive heat and warp the thin metal. You must grind slowly and move around to other sections and let the metal cool.
So that is how I repaired the rust in the Barracuda's trunk. I hope you enjoyed reading and looking at the pictures.