Step 1: Put the cup into a basin, soak it in hot water for half an hour or about an hour, drop some detergent or baking soda into the water to remove the dirt, and then wipe or scrape the dirt with a soft sponge, cleaning cloth or small tree pieces. If the spots are not serious, they should be removed.

The second step, if not removed in the first step, can be gently rubbed with steel balls. Be careful not to push too hard. Just wipe it, don't scratch it. If it is still not ideal, then you should wash the cup and dry it first, and then rub it gently with fine iron sand skin. In general, it can be cleaned.

Step 3: If the above methods are ineffective, don't remove them at all. If the spot is on the outside of the cup, you can dry the cup and stick a nice sticker to cover the spot.