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There is no way to prevent it from forming, short of grinding the surface being welded. The scale is formed when the steel plate is made and when the tubes are rolled and seam welded. Suggest short welds beads of (1") max and let it cool, then go at it again. Wet towels are a good idea.
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Rick
As you slide down the Banister of Life, may the splinters never be pointing the wrong way 
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