Bare spots, uncoated areas on the steel surface, can occur because of inadequate surface preparation. Bare spots may be caused by welding slag, sand embedded in castings, excess aluminum in the galvanizing kettle, or lifting devices that prevent coating formation in a small area. To avoid bare spots, (left) the galvanizer must ensure surfaces are clean and rust-free after pretreatment. Small bare spots can be repaired, but if the size of the bare spot or total number of spots causes rejection, the parts may be stripped, regalvanized, and then re-inspected for compliance to the specifications.
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On some case, a bare spot may be desired to prevent a sealing surface from being Hot-Dip Galvanized, in the instance that machining after HDG may not be feasible either by cost, time, or design limitation. Is there a proven way to achieve a perfect (or near perfect) Bare Spot?
Hi Jack, Masking products are used for this purpose. They are not 100% effective, and may require some minor cleanup after HDG to ensure the area is free of HDG. For more information: https://galvanizeit.org/knowledgebase/article/masking-products-for-galvanized-steel