Top Quality Marine Bolt Supply 18-8 Stainless Steel Finishing Cup Washers (Size #12) These quality stainless steel #12 cup washers are available in convenient packs of 100. Refer to pictures for individual size spec./dimension information. Finishing cup washers are countersunk to give flat and oval style screws that nice finished look. Type 18-8 stainless steel is most common. It has a high resistance to rust and corrosion and may be mildly magnetic.