OK, the MIG wire that I couldn't get to work was 5356. The filler rod from the welding store was ER4043, and worked much better.
My welding book recommends Zirconated electrodes, but I think newer practice is to use Lantanated for aluminum, and I think that is what I used when I did a bunch of frames out of rectangular tube. For 1/4" aluminum, the book recommends a 5/32" electrode ground to a very wide point, 200-300 Amps, 1/8" filler wire, 10 IPM weld speed, a 1/2" cup and
25 cu.Ft./hour gas flow. You can reduce gas consumption with a gas lens.I have a Lincoln Square Wave TIG 300, but I've never done 1/4" thick parts. That rectangular tube was about 1/8" wall thickness. I preheated the work by playing the arc over an area for about 30 seconds and watched closely for the very subtle signs that the metal was heating up. One trick was to melt a little bead of filler metal on the surface of the workpiece, and when it begins to sag from heat conducted through the workpiece, you are nearly ready. Cleaning the work and filler wire with acetone before starting seemed to help, as well as wire brushing the weld area.
Jon