thinest sheetmetal that can be welded?

I'm starting on my first bike and I don't want to use a lot of bondo. I know bondo is the normal method of body mods, but I've had bad experience with projects that I have acquired having TOO MUCH bondo that cracked and fell apart. My situation is that I want all the rolling curves that bondo can create but using sheet metal to get them, and just using bondo for the minor details. I got a 20" stingray frame that I have been reworking for a while, getting it straight and all. It was in pretty dinged up shape. you get what you pay for and 30bones got me a dinged up frame both 72 spoke wheels and the seat, so I'm not complaining. I'm thinking get the thinnest weldable sheet metal that wont fight when I start to curve it to conform, so what do I get; 18ga, 22ga, 26ga? Will Zinc coating mess up the weld? what ever it is I have to be able to mold and flex it with out hammering it all through the night. What can you guys tell me on this one?