Dark color, you can use a brush+neutral detergent+a small amount of water to brush a small area. After washing, it is best to completely support the hat with paper balls and lay it flat, and dry it in the shade. If you can afford a hat holder, try not to put your hat in the water. The brim of some hat brands is made of special cardboard.

Light color, if the trace is obvious, it is recommended to brush it all, so as to save the color difference after brushing. After brushing, wrap the hat with a layer of toilet paper to prevent the watermark from appearing. Other methods are as above.

There is a plastic bracket on the forehead (hard top, the forehead is not very soft), so be careful not to bend the forehead when washing to prevent the plastic bracket from being deformed beyond repair.

If the hat contains wool, it's best to fill it with a hat after washing. Note that if you use paper balls as hat holders, you must pay attention to the fact that the paper balls can't fade. The simplest thing is to completely seal the ball with adhesive tape to reach the maximum size of the hat to prevent it from shrinking after washing.

Dry in the shade because many hats are washed with water, the dye will change color after exposure to the sun. Similarly, many hats with sweaty heads are easy to change color after being exposed to the sun. After decoloration, it is almost impossible to repair except for re-dyeing.

If there are still traces after washing (usually at the root of the brim, or at the position of a circle of sweat belt), congratulations, the hat may be soaked in water or sweaty, left unattended for a long time, and faded after exposure, which is basically beyond repair.

In addition: when buying detergent, be sure to see that there is no bleach component, otherwise you will be at your own risk. Secondly, there are special hat cleaners on the Internet.