Mix 2 tbsp. of baking soda with enough water to form a thick paste. Add more baking soda if you have large sweat stains.
Spread the baking soda paste over the sweat stains on the hat with a butter knife.
Rub the paste with an old toothbrush. Pour 2 tsp. of white vinegar on top of the paste and continue scrubbing with the toothbrush.
Rinse the baking soda from the hat with cool water. If the stain remains, repeat the cleaning process.
Place the hat on a table to air dry after removing the sweat stains.
Talcum Powder and Lemon Juice
Sprinkle talcum powder on the sweat stains, completely covering them. Allow the powder to absorb the sweat from the hat for 15 minutes.
Brush the powder from the hat with your hand.
Combine 1/2 cup lemon juice with 1/2 cup water. Pour the solution over the remaining stains. Allow the mixture to remain on the hat for two hours.
Rinse the lemon juice from the hat with warm water. Launder the hat normally according to the manufacturer's instructions.