(Not sure if that's the right word.)
I needed to make a new moisture trap for my air drier. (dryer?) Instead of playing around with something automatic I decided to make a simple aluminum tank that threads on with a manual drain in the bottom. I used a short piece of piece pipe, and turned a press in cap for both ends. Then I pressed them in. I wiped the assembly down with acetone, and hit it with clean brush. Once I got the settings right (ok functional) the first cap welded on and looks like a pro did it. I was pretty pleased with myself. I flipped it around and had all kinds of issues, and wound up having to use filler to keep from melting back the pipe. Blech! Upon "final" inspection the first cap looked great, but the second cap had two places it looked like I might be able to have a leak (could have been welded inside, but it didn't wet out to were I could see it.) I went back over it with a dab of filler on the two "bad" spots and then circles the whole thing just making it flow and it looks decent, but over welded.
That's it. After the first cap I was prepared to come on here and brag about how easy it was, but after the second I almost didn't post at all.
I think I might have forgotten to clean off any cutting oil before pressing in the second cap. I didn't use any cutting oil on the first cap. It threaded nicely without it. There you go. My fault. In complete prep work most likely.