actually, sunscreens are made of different ingridients, so it is better to get a brand that is known for good sunscreens and la roche posay is one of them. Better ones contain chemical sunscreen such as mexoryl and tinsorb.
sometimes ingridients can interact and weaken each other, so it is better not to layer different types of sunscreens...
also, to achieve level of protection that is indicated on the box, ou need to apply quite a thick layer (its ok to put one layer and then another in 15 minutes, after the first one absorbs) - thin layer won't really give you much protection.