Does sunscreen need to be applied in advance to be effective?

I used to often hear people say that sunscreen should be applied half an hour before going out to be effective. , but in fact this statement only applies to "chemical" sunscreen. Physical sunscreen does not need to be applied in advance. However, physical sunscreen will fall off due to sweating and wiping, while chemical sunscreen will have diminishing effects over time, so no matter which type you use, you need to reapply it! In addition, most modern sunscreen products are two-in-one, so there is actually no need to apply them in advance! Just remember not to apply too little!