Jeenkies! Warpage!

[astroracer]

New HOT welds will make your wet rag sizzle and pop! No water will be pulled into a 1000* weld... Like R_T said it will just boil off. The rapid cooling forces the sheet metal back to it's (nearly) original shape. You definitely put too much heat into the steel. All of your welding should be done as I said in my first post. TACKS ONLY... and... 18 gage will warp just as quickly as 20 so changing gages will not fix the problem.
As an example:
When I filled the tailgate handle on my truck it took me almost three hours to tack it in. I would do a half dozen or so and then lay a wet rag on the patch and go do something else for a while. Get 95% of the latent heat OUT of the panel before you do any further welding. It can be warm but you should be able to comfortably lay your hands on it. Waiting 2 or 3 minutes between welds isn't enough. Without cooling ,the sheet metal is just getting hotter as the heat spreads into the surrounding material and all you did was make a bad situation worse. The hot metal has to expand somewhere and that is what made the bulge...
Mark