We started out with a Lincoln SP170T MIG welder but has since almost exclusively switched to TIG welding using a Miller Diversion 180. It's a slow process but we feel more comfortable TIG welding indoors, have finer control, and can welder a variety of different materials.
The steel table frame was clamped and TIG welded. Previous attempts, Work Table #1 ~ #3, were tack welded before welding of opposite corners and seams. Yet, warping was still very noticiable and resulted in frame to tabletop misalignment. For iteration, the welding clamps were kept on through out the welding process and that seemed to have greatly reduced the frame warpage issue.