A method for preventing osmotic blistering and other defects in the region of a flex line in a spirally-wound sheet filtration element includes applying an effective glue coating as a sealant to the feedstream side of a sheet in regions where a potential for blistering or layer damage exists, such as a fold line. The glue is applied to the inside of the fold, a spacer is positioned at the coated flex line, and the leaf is refolded and wound with other leaves to form a filter unit, such as a UF, NF or RO filter unit. The coating seals surface defects occurring during the fabrication process as well as the surrounding region, and the cured coating strengthens the region so blistering and delamination do not occur.