Any experienced gas welders know if all types of aluminum can be gas welded? Have some 16 gauge scrap I've been trying to practice on but cant get the metal to flow, not even to tack the corners, can see the pieces come to temp but then surface just goes a dull grey and becomes mushy, kind of looks like slag on the top of a smelter when melting aluminum. Using a oxy/acet, cobra torch, tip size 00, have cobalt lenses, have tried neutral flame, oxidizing and carbonizing, all with same results. Have cleaned with stainless brush, 240 sand paper and acetone, looks just the same as uncleaned. No flux, cant even get to the point of using filler rod. Only thought I have left is that some there are types of alloy that cant be gas welded?