Single 2.5 inch Diameter Backer Rod and Craft Foam in Grey. Shade of grey will vary. Allows Easy Control of Sealant Application Thickness Won't Adhere to Sealants. Closed Cell Backer Rod is a round, flexible, polyethylene foam. This is also used as a thickness control device for elastomeric and other cold-applied sealants. Great for many crafts too!. Safe and non-toxic.