Out of position welding of aluminum. TIG? Or MIG?

Gentlemen,
I have an old (1978) 30 foot aluminum dump trailer ("gravel bucket) that is in need of some welding TLC.
It is shot through with fatigue cracking. While I do not remember the exact alloy, it is one of the "imminently weldable" flavors.
Most of the fatigue cracks are in the sides. Therefore, all welding to these cracks will be vertical.
Additionally, some of the reinforcing gussets welded between the bottom of the floor and the aluminum cross members are either cracked or missing. Therefore, there will be a considerable amount of overhead welding.
An added complication is that I have no experience welding aluminum.
The only bright side to this picture is that I have a couple of excellent machines. The first, a Miller Syncrowave 500 (625 amps of maximum output!) has virtually all the bells and whistles. In fact I believe it has everything EXCEPT a sequencer.
The other weapon is a Millermatic 250X with Miller spool gun.
I have been told on good authority that over the decades, especially in dump trailers, the aluminum sucks up all kinds of contaminants, which you cannot adequately remove by merely cleaning the metal.
Although thorough cleaning is obviously mandated, "the word on the street" is that, to get a good weld, you have to weld a bead, grind it out, weld it up, grind it out - repeatedly - until FINALLY the goop will boil out and you'll finally get a good weld.
The above was communicated to me by the honcho who works for the factory. Therefore, I trust this to be good information.
Because of this, it would seem to me that the better option would be to TIG weld as much of the repairs as possible. My understanding is that, especially with the pulse and other controls on the Syncrowave welders, and provided you dial it in correctly, I should be able to sit there and stir the puddle without burning through and glopping a big wad of aluminum onto my boot.
Has anybody been there and done that? Obviously, I'm not gonna just strike up either machine and produce perfect welds. As a matter of fact I have virtually no experience (as yet) with either machine even with steel. So the plan is to start practicing rather intensively.
What I'm hoping to get from this post is an orientation by somebody who's actually experienced at doing out of position aluminum patch welding with both processes - and ideally - with these very machines.
I take this opportunity to thank everybody in advance for your sagacious input.
Regards, Vernon
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Have you considered the price of aluminum scrap these days, versus replacement cost of a decent trailer?
Gunner
"Gunner, you are the same ridiculous liberal f--k you ever where." Scipio < snipped-for-privacy@actd.net
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Hi Vernon, If you have a great deal of repairs you might consider removing the box from the frame. If you have a loader large enough it might be worthwile. You could cross brace the open part of the trailer and identify all your repair sites with a felt marker before removal. A die grinder with coarse carbide burrs will help prepping corners and gouging out cracks. The slivers are murder though. A facechield and gloves are mandatory. Good quality vertical and overhead welds are doable with MIG however it does take skill and practice. I had a friend in a shipyard who being a top hand was asked to qualify to do repairs on 2 mm aluminum on a large hovercraft. It took almost a week for him to get the procedures down pat before he could move on to the job. That is not a typo... 2mm with MIG on pulse. Gunner has an excellent point about the value of scrap. Randy

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Randy,
Unfortunately, I don't have the luxury of taking that amount of time to do an off-frame repair.
Some of the old repairs on the trailer need to be re-done. Those welds look like bubble gum in places.
I never had thought about a pulser with a MIG. Is this a requirement for doing vertical welds? If so, it's back to the drawing board 'cause although the TIG machine has one the Millermatic 250X doesn't.
The best plan may be to work the trailer until it cracks into two pieces and then scrap it. I had not thought about the scrap salvage value of the aluminum bed. Maybe I can find a steel bed and put it on there.
But back to the matter at hand. I did not think welding aluminum out of position was THAT difficult. Of course, in my mind, everything is always easy.
VT Randy Zimmerman wrote:

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Some truckers put a 3/16 AR steel plate in the last third of the dump box floor when the box is new. I wouldn't go to the trouble of pulse. It is possible to weld vertical with MIG on aluminum but it takes practice. I have talked to guys who built new boxes welding vertical up by pulling the trigger in short bursts. The MIG weld looks much like a TIG stack of dimes. In this case the owner of the company liked the stack of dimes appearance and so did the truckers.... Go figure. Randy

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Randy,
Pray tell... what is "AR" steel plate. And why do they put it in the last third of the dump box floor?
Tanks! Vernon
<<Some truckers put a 3/16 AR steel plate in the last third of the dump box floor when the box is new.>>
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Abrasion resistant plate takes all the sliding abrasion that wears out the floor when the load is tipped. The back end suffers most from this and since the steel add to the weight the last third is the important part. Also it gives the loader somewhere to drop the Big rocks. Boulders of course are murder on an aluminum box. Randy

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Hello Vernon,
What did you finally wind up doing with this project?
Jim
Vernon wrote:

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welders,
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