Iosso Products Seam Sealer 4 Oz IOSSO seam sealer prevents most seams from leaking. Best for use on cotton, cotton blends, polyester and synthetic woven fabrics. Water repellency will vary with vinyl coated and vinyl laminated fabrics due to seam construction. It will not change the look or feel of the fabric. Ideal for boat covers, awnings, tents, backpacks, etc.