Whether you use a commercially cream or home made cream, the success of sunscreen depends on how it is applied.
The application depends by type of filter protection.
If you use a cream that uses a chemical protective filter will need to be applied approximately 20 minutes before sun exposure, if the filter protection is mineral (zinc oxide or titanium dioxide) exposure to the sun can be immediate.
How much to use?
It appears that most of the times the amount of cream used is much less than the amount required to provide the degree of protection on the label. The recommended amount is generally 30 grams / application for an adult. That means for a good protection, one tube of 150 grams should last about 5 applications.
How often shall apply sunscreen?
Even if you use a waterproof cream, follow the manufacturers advice and reapply it after you get out from water. Many recommends reapplying every few hours or even more often.
When to use sunscreen?
Certainly not just when we go to the beach and not just in summer, but when we expose ourselves to the sun.
Use creams with high sunscreen , like 30-50 FPS, provides a false sense of security, those who use them tend to stay longer in the sun and violate the rule of not staing in the sun between 11 and 17.
After all the above it seems that the best sunscreen is simply avoiding exposure, using natural fiber clothes that cover as much (No wonder in desert areas are used clothes which covers almost the entire body) or if you can’t avoid the sun use the correct amount of sun cream, applied at regular intervals.