Hello DrEmmerich. I can't give you nearly the quality of advice some others here can, but I'll try to give you a straightforward answer.
If I understand you correctly, you've fit up a weldment without cleaning the joints, specifically the oxide layer (shiny), prior to welding. There are a couple of potential problems there. First, if you tack welded the parts without cleaning, then your welds are already likely contaminated. Second, the best way, as Dave stated above, to remove the oxide layer is with a clean stainless steel wire brush. I strongly discourage the use of burring tools, or sanding tools (especially which utilize aluminum oxide abrasives); the removal of base metal is next to impossible to control effectively with such tool, and with aluminum oxide abrasives you will likely embed some of the material into the base metal, thus defeating any benefit by using the tool. Another hazard of using pneumatic tools is that you run the risk of spraying tool or air system oil on your parts.
There is quite a library of cleaning methodology for aluminum alloys in this forum, which can be found easily by using the search function at the top of the page. In the interest of keeping it short, I can recommend that you always clean your joints first with some kind of degreaser, followed by a serious wire brushing. Immediately prior to welding, hit it one more time with the wire brush, and a lot of folks like to use a chemical cleaner, such as isopropyl alcohol, MEK or acetone just prior to welding, which will remove any residual grease or oil. My personal philosophy with aluminum is to clean. clean, clean and when you think it's clean, clean it again! At any rate, good luck and let us know how it turns out.
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