James, ALL of the anodizing must be removed before you will obtain a good tig weld. Aluminum oxide is the whitish substance that forms naturally on the surface of aluminum when it slowly reacts with oxygen in the air. Since alum. oxide has a much higher melting point than pure aluminum (or its alloys), it is difficult to tig weld through a layer of the oxide. This naturally occuring oxide must be cleaned off. Anodizing, on the other hand, is an man-made process by which a protective layer of aluminum oxide is deposited on the surface of aluminum. Chemically it is identical to that formed by the interaction of air with aluminum. The alum. oxide laid down by anodizing, however, is a much tougher and thicker deposit (~1.5 to 2 mils). Unlike the naturally formed alum. oxide, anodized coatings are harder to remove from the surface of an aluminum part. Perhaps the best way to remove it is to power sand or grind it off (carefully so as not to remove too much thickness), wire brush with a stainless steel brush - since the abrasive substance in most grinding disks and belts also is alum. oxide - and clean with alcohol. Even after all this work you may still not be able to lay down beads as nice looking as if you were welding on unanodized aluminum. My reasoning for this is that creating a 2 mil layer of anodizing, for example, requires the chemical conversion of the upper 1 mil layer of the aluminum surface of the part. Then, an additional 1 mil layer is deposited on top of that. Because anodizing involves chemical conversion of the surface layer of the part itself, perhaps some of the alum. oxide may be deeper than 2 mil. below the surface of the part. I'm not an anodizer but have had many of my own parts anodized. Others here with more welding experience may shed more light on this subject.
Jim,
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