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Old 02-03-2013
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Re: rusting stainless

Quote:
Originally Posted by RichH View Post
Your stainless was probably supplied with a common 'mill finish' - the surface is rough, has many mill rolling marks, not polished.
Also, possibly the material was processed with plain steel tools, jigs and fixtures with the plain carbon steel contaminating the surface of the stainless steel.

For rust prevention and corrosion retardation, 300 series stainless has to be supplied in at least a 300 to 180 grit finish to remove processing scars and surface defects of manufacture by the mill followed by or purchased from the mill already 'sanded'/ground; then surface then can be 'passivated' to remove the remaining surface IRON. Simple passivation should only be done with a surface ground to at least that mill supplied 180 grit finish. For best corrosion resistance (rusting, etc.) is to grind all welds flat and planar to the surfaces followed by grinding, then finely sand (down to 600 grit) followed by power-buffing to a mechanical mirror bright surface to remove all scratches and mechanical blemishes, followed by electropolishing.

If you take 'any' 300 series stainless steel that is not mill or shop finished to the above, it will surface rust, especially on boats when in contact with sea water. Thats why 99.9% of the stainless steel you see on 'boats' is 'very shiny' - its highly polished to prevent rust and corrosion.
Rich is exactly right. I did that on all my chainplates, headstay fitting, lifeline stanchions etc. and they looked as good as new for years until I sold the boat.

Buying a buffer - like a bench grinder with extra long shafts - is a great investment if you are working on an old boat - it will revive all but the worst S/S corrosion and keep it looking good. You can get handyman versions from Harbour Freight for under $50 and they work fine. Electropolishing is not that expensive either. It should be mandatory for chainplates and keel reinforcements.
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