Remove sweat stains from baseball hats using a laundry pretreatment product and laundering with a small load of light-colored clothing using ordinary detergent. Cold or warm water wash and air-drying prevents the hat from shrinking. Wool or felt baseball caps require hand washing and drying on a form to retain their shape.
While many people believe machine washing any baseball hat in the washing machine ruins the bill of the hat, this only applies to baseball caps made before 1983, according to CleanMySpace.com. Until that time, manufacturers used a cardboard form for the bill. However, since 1983, manufacturers have changed to a plastic bill form that withstands machine washing.
Wool hats require more care than those made of cotton. Wool baseball caps should be hand washed using cold water and a gentle detergent. Once the sweat stain is gone from the hat, rinse it in cold water and roll it in a towel to absorb as much water as possible. Dry the hat over a balloon, inflated to approximately the size of the head of the wearer. Drying without a form causes the wool fibers to shrink, leaving it misshapen.
Fedoras and western hats are not washable at home. Sprinkling perspiration stains with cornstarch absorbs the perspiration oils and allows them to be brushed away. If the cornstarch does not remove the stain, the hat requires professional cleaning and blocking.