thanks!
The rounded mallet does work, just a lot more work to smooth out and prep for paint. If I were to try it I would definitely use heat while pounding it. You're right though, it would probably take quite a bit of bondo to smooth everything out.
The cut,weld, grind method works good, if you have access to a half way decent way to cut it and a decent welder. The better the cut, the better fit you'll have, which means the better weld you will get, which means the easier the clean up. lol.
I actually TIG welded one side of my tank and MIG the other side. IMO MIG worked better. The TIG worked great, however, if you cut it with a plasma cutter you really need to clean up the cut edge good. Otherwise it really makes TIG welding a pain. The plasma edge causes a lot of porosity if not cleaned up. I cleaned mine really well and still ended up with some porosity which I had to re weld and re grind a few times.
With mig it doesn't really matter. The metal is so thin that it basically just melts it together and I didn't have any porosity issues. When you MIG it, you basically just lay a line of tacks. Trying to lay a bead just blows right though. This tank was like 20 or 22ga, which most are, but if I were to build a tank it'd be 18-16ga.
You also need to be super careful when grinding. It's way to easy to see a low spot and just go after it with the grinder. You're better off filling it back in and re-grinding. You can get yourself in trouble fast and grind away too much.
It's mostly done now, I want to leak test it yet to make sure there's no pin holes that I missed. Then it's onto body filler and all that funn jazz.