You will need a light hand and work carefully if you want to restore a felt hat that is an antique or expensive or if it has great sentimental value. Felt wool and most other felts are difficult to clean with household products, and they may damage your hat. Deep stains caused by sweat or mould are better left for a professional. But on the bright side, there are a few tried-and-true methods to help you restore a felt hat.
Brush your hat to remove any loose dirt and dust. Use a soft-bristle hat brush. Gently brush the crown and brim of your hat counterclockwise, and brush the top of the hat toward the back, suggests Renton Western Wear. This will loosen debris and lift the nap of the felt.
Remove stubborn lint or fuzz balls with painter's or masking tape. Wrap the tape around your hand with the sticky side facing up. Apply it lightly to the area where you see lint, and then quickly remove the tape. The lint should stick to the tape. You can also use your vacuum cleaner if it has a brush attachment. (See reference 2)
Clean stains with baby shampoo and water. Put one tsp of the shampoo in one cup of warm water. Pour about a tbsp on the stain. Massage it gently into the stain with a hat brush. This should work for light stains and spots but may not effectively clean an old or deep stain. Some small, stubborn stains on felt hats can be sanded with a light-grit emery board. (See reference 3) Very carefully and gently rub the spot, making light, even strokes going in one direction across the stain. Do this very carefully, and do not use a lot of force; you want to avoid making a bald patch or ruining the hat. After cleaning, steam the area using a teakettle, pot of boiling water or steamer. Hold the area of the hat you just cleaned in the steam for 20 to 30 seconds to restore the nap.
Reshape your felt hat by using steam; this will make the hat soft and pliable. Hold the hat over steam for about 1 minute. Do not get the hat soaking wet. Reshape the brim as needed, or dent the crown using your hands until you are satisfied with the shape. Allow the hat to dry on a hat form. If you do not have one, use something like a flower pot or bowl, anything that will not stretch your hat and allows it to dry without resting on the brim. Never use an iron or blow dryer on your hat; this could cause it shrink and shrivel.
Spray on a hat-stiffener product especially made for felt hats. This will allow it to keep its shape without residue. Many people try to take a shortcut using starch or hairspray, but that is not recommended and will cause build-up and residue over time. Invest in water repellent and stain protector spray made for felt hats once you have restored your felt hat to keep it looking good.