Its an oil type primer available from paint section of hardware stores. Its not cheap though, but it spreads out so much as it 'penetrates' that a small amount goes a long way. It leaves a glossy surface which looks quite nice but it scratches if not painted over. We reguarly coat the underside of our quad bikes and trucks as they are driven through salt water and acid every day. About 3 years ago we painted some badly rusted girders in the shed with it - they still look the same as the day they were wire brushed & painted, rusting process completely halted.
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