Don't start on sheetmetal.....you WILL burn through it. Learn on thick stuff. By the way, the sheetmetal is probably 18 to 20 ga.
Thanks Russ I have some thick stuff but I also have some parts like the original tailgate and front bed panel that were so beat up that I bought reproduction ones because it just wasn't worth the time and effort. I also have some wrong (too small) patch panels that I bought that I can try on. My first go will not be on the stuff I want to save. You have my word on that. I will try first on the stuff that I don't plan on reusing.
Ask your bodyman for a bent-up car hood or any large body panel from a collision job. They usually have some out back, in a scrap pile. Practice welding on those, modern car hoods are thin. I use the fine ga. Wire, .025, on light sheet metal projects.
Please Excuse My Off Topic Excursion....... ----> Did you say tailgate? Re-purpose that sucker, something like this: