Welding repairs on an aluminum water tanker, different alloys present?

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One of the guys at work is repairing a semi trailer water tanker.  We think that some of the aluminum is different to the rest of it.  The main runners of the chassis look to be extruded and because of this we think is a different grade of aluminum compared to the ally that butts up next to it.  The welding is not that pretty anyway because of all the years of dirt and road grime that would have been impregnated into the ally.  It was cleaned as best as possible, but in the real world sometimes you just can't get it perfectly clean.  We have been using the mig on pulse as we would normally do.  On the bench with a new pieceof ally it welds like it should.  Under the tanker, ha, lol it's a different story.  We can weld the actual tank part, but as soon as it hits what looks like extruded ally it just goes bad.  There is a change in the arc and sound etc.If the extruded ally is different what would be the likely grade that might be used under a water tanker truck as part of the frame/chassis.   Then what filler to use on that?  We are on 5356 now, maybe 4043 should be used?
Reply:Pictures are blocked on my work PC, so if you posted any, I cannot see them. I'll hazard a guess nonetheless: when you talk extruded, you realise that most of all ally is extruded, correct? That's mostly the easy part, as most extruded aluminum alloys that are widely commercially available are weldable as well. If there is any cast ally present, you need to be more specific. Based on this, I'd say 4043, or if that cracks, 4047. I'm not a huge fan of 5xxx fillers unless there is a good reason for (and when you weld specific alloys).What do you mean by "cleaned as best as possible" ? abrasive/sanding, or only wiped clean ?
Reply:Its been my experience with welding any aluminum that has been exposed to water corrosion thatyou will need to get that removed back to clean raw parent metal before attempting to weld it with either type of welder.I like a wide fluted burr and die grinder for that job.....unless you get it all out your weld will be terrible. Typically I use 4043 but since 4943 has been introduced I really like it over 4043....  Miller Dynasty 350Twenty Six HammersThree Crow BarsBig Rock
Reply:I'd use 5356 wire and the extrusions are probally 6061 and the tanks would be 5052  get yourself some aluma prep acid it will take most  of the contaminants and then give it a good heat with a torch to get all the water and stuff out let it cool then wire brush and weld