I live in Greece which has a pretty warm climate, tend to do more trail running than hiking these days, my experiences apply well to hiking as well though.
Found that although the OR Sun Runner offers good sun protection the choice of material and DWR is a terrible flaw.
I can dunk the cap in water and it’ll be dry to the touch in under 10 mins while wearing it, might sound good for it’s wicking performance but for keeping you cool it’s the opposite of what you want.
I’ve tried buffs but just can’t get a consistent neck, ear position that protects them.
Found if i wear the buff under the cap my head feels like a boil-in-the-bag chicken for 10 mins but it’s then cooler than just wearing the cap alone.
Not as cool as wearing nothing, but it’s the best compromise i’ve found yet between cooling and protection.
For my upper torso i tend to wear a very thin long sleeved dri-fit top, like the OR cap it wicks too well really, so i’ll slip on a light coloured cotton tee-shirt over it, the baggier the better.
For the legs i wear shorts and slap on loads of sun block.
For sun block i’ve found kids sun block works best, it tends to apply thick, cover well, offers good protection and stays put.
I’ll put it on my face, neck, ears and legs and i’ll apply enough to do a good casper impression.
Don’t really care if it takes a good bath to remove it, i’d sooner go to bed with some left on than have it come off and leave me burnt.
Couple more findings that aren’t exactly what the op asked, but hopefully some might find useful.
1/ Get out as early as you can, if you can start just before sun rise you’ve usually got 3 or 4 hours of clear trails and even in the peak summer months it’s a bit fresher in the mornings.
2/ Find some shade for the mid day, or slow your pace down dramatically if that’s not possible.
3/ Don’t pass a decent water source without soaking your clothes, be careful with headgear though as if the water is contaminated it is possible to get ill, i look at and any soaked headgear water source as if i was going to drink it.
4/ Make your sweat work for you, you really want to hold onto every drop for as long as you can, this usually involves more than one layer, sounds pretty much the opposite of what you’d want to do and you do boil for a while till the sweat builds up, BUT once you have that moisture evaporating through the layers it does eventually feel cooler.
Not as cool as no layers, but as i said above it’s a decent compromise and i find it cooler than wearing just a single layer.
Other than that, i tend to stick a couple of water bottles in the freezer the night before, i’ll use that water to sip (i tend to get stomach cramps if i drink too much cold water) and douse my head from time to time.
I find the running type collapsible water bottles good for head dousing, as i can squeeze the bottle to get the pressure i want and pinch the drink nipple to get a nice stream of cool refreshing water on my noggin.
For the longer runs (over 3 hours) i’ll also make up a drink to replace the salts, usually use around 1 teaspoon of bicarbonate soda, 1 teaspoon of salt, i’ll then i’ll squeeze whatever fruit we have in the house, or use fruit juice to give a little taste.
It’s basic but i do find it helps me, both during the run and recovery, should note that i don’t just drink this mix, i’ll usually take a sip from the mega-mix bottle then sip straight water for a few times, can’t say as i measure it, but lets say 1 x mega-mix sip for every 4 water sips.
If i feel i’m dehydrated when i return home i drink a big glass of milk, again i’ve found milk seems to hit the spot better than water alone when it comes to rehydrating.
As you can probably see that’s a fair few water bottles, i find i usually drink around 1 litre an hour when running in 30c plus heat.