The tighter the gap the easier it is to weld and also the less warpage you will experience. If you can not get full penetration on sheet metal with no to little gap you are welding wrong, turn the weld up heat wise.
IMO those "special" butt weld clamps leave a huge gap, way too big for proper butt welding. The gap left is almost twice as big as the filler rod!!!!
If you need to clamp the panels together and do not have clamps, try this method. Works great giving you a no gap joint with only a few small 1/8 holes to weld shut. Much better then a .040 gap down the entire seem.
http://www.chevelles.com/forums/showthread.php?t=302119&highlight=cleco+butt+weldWhen welding with a MIG you do NOT weld in beads. You weld a seem shut with countless superhot tack welds. Also contray to popular belief, a hot weld produces less warpage in the panel. A hot weld takes less time to generate full penetration versus a colder weld taking more time which introduces additional heat causing warpage.
