The chemists found that one hour after applying sunscreen the protective effect is greatly reduced, because the UV filters sink into your skin. And worse, it can cause more damage when deep in the skin than not wearing sunscreen at all!

Right after application the sunscreen sits on the surface of the skin, stopping the UV rays penetrating to the skin cells.

After sunscreen has been on the surface for a while, it can penetrate into the skin. UV and sunscreen react to form free radicals that can cause skin cancer. These reactions should be reduced when you reapply the sunscreen. Than the UV should be blocked on the skin surface rather than reaching the sunscreen at the deeper skin layers.
The researchers recommend that it may be necessary to reapply sunscreen every two hours or more often.
Another recent study says if you rub the sunscreen in, it accumulates in lines and sweat glands and does not offer much protection.
What is my verdict?
1. Make the sunscreen the last line of defence, not the first.
2. Reapply it once an hour when in the sun.
3. Do not rub the sunscreen in, just apply a generous amount onto your skin.