Last night I was welding these landing gear leg fairings together (1937 cabin Waco) - 3003 .020 to .040
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and here is my checklist:
clean aluminum with a good wipe with 91% isopropyl
brush with clean ss toothbrush
Get a good fit
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mix flux with clean bottled water (Super Primo flux, in this case)
neutral flame, #2 tip, + slight feather, inner cone is 2-3X metal thickness
I try to parent metal/fusion weld the first tacks.
Then I go over that with a good long "tack."
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I add to the tack, and then using the step-back method, whereby I start back from the weld and then weld to it - either forehand or backhand ...
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and thus continue to the fully-welded part.
(ps, the "step-back" weld is very old and is used to limit distortion on gas-welded parts.)
thus:
top side:
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and the root:
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end