Sealing washers have a layer of Neoprene bonded to a raised metal washer to form a seal that is strong while also being soft and resilient. They can be used with fragile materials and to provide extra cushioning under a nut or the head of a bolt or screw. Sealing washers are effective in stopping leaks by sealing bolted joints. Washer size refers to its nominal hole size and is based on screw size. Its outside diameter (OD) is always larger. Size is usually specified as a number (#10 for example) for smaller sizes, and in fractional inches for larger sizes (like 1/4"). Some sealing washer sizes are offered in small and large OD versions.