Hello Duncan. I need a little more information, but I weld on 6061 T6, 3/8 inch plate, with the mig process. I do use a preheat, and make sure the base metal is very clean. I use ER4043 filler metal. Hope this helps answer your question. If you have different variables than what I stated, someone else might be of more help. What exactly are you working on? Mine isn't code work, so more input might be needed for your application.
Slagman

If you are looking for a color match after anodizing, higher weld ductility and higher weld strength if may wish to consider ER5183, ER5356, ER5554, ER5556, or ER5654.