One of the ways to differentiate "Good" stainless steels for use on boats, from inferior metals, is the use of a compass or magnet. Good stainless steel is not magnetic.

Type 303: Similar to 302, but with phosphorous and sulfur added for relatively easy machining. Often used in fastenings.

Type 304: Has less carbon than 302 or 303 which reduces carbide precipitation when welding. Somewhat greater corrosion resistance than 302 or 303.

Type 304-L: Extra low carbon version to further reduce carbide precipitation in weld zones. This type should be specified where the piece will be a welded fabrication, even if used belowdecks. All above cautions apply. Types 304 & 304-L will develop a rust film with exposure to salt water. Below the waterline they corrode freely. High stress will accelerate this corrosion. Types 304 and 304-L should NOT be used below the water.

Type 316: Contains molybdenum for higher corrosion resistance. Used for fastenings above water. Will not readily develop a rust film. Very resistant to pitting in salt air.

Type 316-L: Extra low carbon version used for welded fabrications to prevent carbide precipitation.

Types 316 and 316-L are much more corrosion resistant than types 302 or 304. Still, 316 should NOT be used as a fastening material for any structural part of a wood hull, even above the waterline. In aluminum boats 316 is sometimes used as a seacock material below the waterline. Isolation of the two materials must be thorough. Type 316 can be used as a shafting material below the water but must be protected with zinc anodes.

Type 321: Stabilized with tantalum and columbium to prevent carbide precipitation. An excellent choice anywhere above the water. Welding is easily accomplished.

Type 347: Stabilized with tantalum and columbium to prevent carbide precipitation. An excellent choice for use above the water. Easily welded.

Aquamet and Aqualloy: Two different trade names for essentially the same series of stainless steel. These shafting alloys are corrosion resistant and weld-able, but expensive.

Type 400 series: These have NO place aboard a boat.

When polishing stainless steel, make sure you don't stain surrounding gelcoat. Always use a nonabrasive polish (Comet or Bonami are too coarse). Rust stain removers such as West Marine Metal polish, Starbrite metal polish, Boeshield Rust-Free and others are a good way to start. Then go to a polish - like 3M Metal Restorer & Polish, Nevr-Dull, Meguiar's, Starbrite, West Marine and Flitz metal polishes. Bronze or steel wool will quickly destroy the steel's protection against rust. Steel wool should never be used on a boat anyway since the small particles will embed in the boat and cause rust specks on those areas. A new paste called Wichinox from Wichard-USA repassivates stainless that's lost it's protective skin.