Method 1. Wash off with water. Or brush with 10% ammonia solution, wash with dilute acetic acid solution, and finally rinse with water.
Method 2. It can be wetted with 10% citric acid solution, and rinsed with water after one hour.
Method 3. Firstly rub with kerosene, then rub some dilute acetic acid, and finally wash with water to remove.
Method 4. Sprinkle a few drops of alcohol on top of it, then rinse with warm water and soap.
Method 5. Immediately wipe off with turpentine. New stains can be rubbed in the water.