Method 1: If your watch can be waterproof for more than 100 meters, you can directly put your watch in salt water and soak it for half a day. Then take it out, and you will find that there will be black sediment at the bottom of the salt water, and those are the black objects from the watch chain. Then just like you brush your teeth every day, squeeze a little toothpaste on the watch, and gently clean the seams on the watch chain with a toothbrush.
Method 2: Put your watch into the ultrasonic cleaning machine for all-round cleaning of the stainless steel strap of the watch. The ultrasonic cleaning machine can strip and disperse the dirt with 40,000 high-frequency oscillations per second, even the stubborn dirt can be removed.