When should I apply sunscreen for sensitive skin, and how often should I reapply it?

My skin is sensitive, so I always struggle when choosing a sunscreen. In particular, as I am about to undergo treatment for melasma and age spots, I am looking for a product that can be used without irritation. I recently heard that the Liviheal Exosome Sunscreen is free of harmful ingredients and does not sting the eyes, so I really want to try it. My questions are regarding the sunscreen When is it most effective to apply Also, on days with heavy exposure to sunlight How often should you reapply it to safely protect your skin Since I am looking forward to skin soothing and anti-aging effects, I would like to know the correct way to use it. I would like to hear the experiences of those who have actually used it or advice from experts.

If there are any other good products, please recommend them.