I forgot to add that the factory guy said the same thing Ernie does below. After so many years of use the aluminum, which is very porous, sucks up a world of grit, grime, diesel smoke and other shit that is NOT ALUMINUM.
He said that once I start welding up cracks I'd likely be wringing my hands in despair over the gloppy porous welds.
But he urged me to grind 'em out and weld 'em up again and again. And that finally, the crap would fly up to heaven where it can be useful to God's work of depleting the ozone layer, and I'd finally get a good weld.
Frankly, I haven't yet made a single aluminum repair. I'm up to my neck in suspension and axle problems...
If you get good at fixin' yours why don't you come on down and fix mine??