I think if you use a sunscreen high in ZnO you don't necessarily need to reapply.
It is my impression that it is essential with any sunscreen, even the most photo-stable. It has been discussed quite a bit on these forums. Needless to say, not many people do this because it is exceptionally inconvenient, especially for anyone that wears makeup or concealer. The first reapplication would smear the hell out of their makeup.
This type of product may actually make reapplication at any interval feasible, and in this way may prove to be the most effective type of sunscreen in terms of the delivery of the sunscreen agents.
It seems to me that you can have the best sunscreen available, but if you only apply it once, then it seems that it may be little better than doing nothing.
Re-apply sunscreen approximately every two hours
http://www.aad.org/media-resources/stats-and-facts/prevention-and-care/sunscreens
Recent research at the University of California Riverside indicates that sunscreen needs to be reapplied within 2 hours in order to remain effective. Not reapplying could cause even more cell damage than not using sunscreen at all, due to the release of extra free radicals from those sunscreen chemicals which were absorbed into the skin.
http://www.ncbi.nlm.nih.gov/pubmed/17015167
Reapplication of sunscreen is just as important as putting it on in the first place, so reapply the same amount every two hours.
http://www.skincancer.org/prevention/sun-protection/sunscreen/sunscreens-explained
Reapply sunscreen every two hours
Forget all-day protection. Sunscreen wears off after just a couple of hours. So, make sure you lather on another golf ball-sized glob every two hours.
http://www.mdanderson.org/publications/focused-on-health/issues/2011-may/sunscreen.html
Reapply sunscreen at least every 2 hours...
http://www.fda.gov/f...s/ucm258416.htm