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Is it a viable practice to not wear a shirt, as long as you smear your body with sunscreen, if you plan to go hiking for hours (assuming reapplication)? Or is it advised to wear a shirt at all times and apply sunscreen anyway?
I'll be doing some hard grade mountain walking up to see the steamers here.
Assuming you're reapplying the screen often (sweat washes it away) and it's a quality one, then it would do for what matters sun protection (although you should still properly cover your head with a hat or a bandana or whatever).
That being said, I would use a shirt anyways; to protect your skin from the abrasion of the rucksack, to absorb some sweat, and to protect your guts in case of wind or light breeze that could somewhat affect your digestion, in the long run. :)
According to this health-related website, you should still wear a shirt:
You should not think of sunscreen as an alternative to avoiding the sun or covering up. It is used in addition. Sunscreens should not be used to allow you to remain in the sun for longer - use them only to give yourself greater protection. No sunscreen is 100% effective and so it provides less protection than clothes or shade.
As Paul says, think of sunscreen as in addition to other protection methods. So while you can go without a shirt, it's ideal to also wear a shirt and use other protection methods like a hat.
As for whether clothing is always better, that in large part depends on the clothing. Some clothing provides good sun protection while others (think shirts with looser weaves) provide less protection. In fact a piece of clothing's sun protection factor is called its "UPF" rating. Many outdoor retailers sell clothing with a stated UPF rating.
No matter what you decide, you should re-apply every two hours to make sure you get full protection. This is advice for everyone and particularly important when you are active.
If you are a person who burns readily then sunblock at all times plus long sleeve shirt and hat are your only answer.
However the danger of sun, IMHO is overrated, compared to the other hazards of life. At one point I used the World Almanac for figures: Fatal skin cancers kill about 2 people per hundred thousand per year. So skin cancer has about the same risk as getting murdered in a civilized part of the world, or about 1/4 that of being murdered in the U.S.
Given also that skin cancer risk is greatly increased if you have had even one serious sunburn, the risk seems to be associated with burning, not the exposure.
If you are more comfortable without a shirt, do so. If you have a sensible prep plan for a couple months ahead of time, you can skip the sunblock.
In passing: In 35 years of working outside and canoeing, I have ended up using sunblock on nose and ears, and occasionally full face at higher elevations.
