metal prep goes over bare metal. It oxidizes bare steel to a dull gray finish. I got a quart bottle from any old auto parts store, and put a pump spray top on from an empty laundry stain remover. If you have a tough rust converter type paint, it'll stick to that too, just rough it a bit first.

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'82 300SD - 361K mi - "Blue"

"Good judgement comes from experience. Experience comes from bad judgement."

listen, look, .........and duck.