A straw hat stiffener is a substance that seals, hardens and reinforces the outer structure of straw-like materials used in the millinery trade, such as sisal, paglina and sinamay. Straw hat stiffeners are either lac-based, some form of lacquer, or a solution of polyvinyl acetate glue and water.
Milliners and suppliers of millinery materials refer to straw hat stiffener as sizing, hat sizing, sealant or matte interchangeably. Mass producers and private milliners often apply a layer of stiffener to hats prior to any sales, which helps to maintain a hat's shape and protect it from the elements after a customer purchases it.
Lac-based stiffener, or shellac, is essentially a thin laminating material produced from a lac bug's secretions. This is a thinner version of the same shellac that is sometimes used to finish wood. Straw hat stiffeners made with lacquer are either another from of shellac sold as lacquer or a product derived from the sap of the lacquer tree.
A mixture of PVA glue and water applied by brush or spray bottle can stiffen straw hats as well. Most recipes advise one part PVA to four, five or six parts water, depending on the desired level of stiffness. A thinner solution works best on straw, especially sinamay.