For light or dark skinned persion, what is a good value for hte SPF.

Typically an SPF of 15 or 30 is recommended.
But l see that you catagorized your question
under Surfing. So, with that said, you should definitely
use an SPF of 30 to 50. And you should apply it
every few hours or so.

There are a bunch of products out there.
Go for the sweat and/or waterproof kind.

The higher the better, especially if u r out surfing as the water reflects & magnafies the sun is radiation.

Absolute min I'd suggest SPF15.

You'd also be best to buy one that is water resistant, as this will help keep it on longer, but always come in every few hours & put some more on, cause surfing or playing in the surf washes it off fast.

Put it on ur face & around ur eyes 10 - 15 mins before ur go in the water, this lets it soak in a bit better, otherwise it runs a bit & can sting when it gets in ur eyes.

***note - the SPF level is not very informative. for example a spf 30 isnt twice as good as a spf 15. but still the higher the number the better the protection.****

Bet the brand Sunblock SPF 90. Trust me they make it. l use to work as a Jr lifeguard back home in Hawaii. That is all I've used and it works. Hope you can find some.

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Probably around 30-50 SPF

spf 15-25 works the best but you should always reapply

You do not need anything more than 30 spf. Just make sure that the sunblock you're using is waterproof. If you want to keep your tan, you should not stay out in the sun for a very long time. Just short pops of an hour or two everyday. Trust me, l live in Hawaii.