Cotton and sun protection

It depends on a couple of things. First is how tight the weave is on the shirt. I would think an oxford-style shirt that was not overly worn down would be fairly good. The other factor is the color with white providing the least protection, all the way up to the best being black. Also, shirts that get wet tend to lose a significant amount of their SPF protection, so sweat soaked areas exposed to the sun will provide weak protection. The general rule is that white cotton T-shirts are SPF 4 and up. Tight weave darker fabrics can exceed SPF 10. Specially designed sun protective materials can go up to SPF 30.

I have a white Nike UV T-shirt (rated at 30) that I used for my very sensitive skin last summer during a week-long hike in the High Sierras. It was great. I supplemented this short sleeve shirt with the cut off arms of an SPF 30 rashie/water shirt that I slipped on and off as needed. The system worked well, though next time I will try to sew in some elastic to make sure the arm gaitors stay up consistently.

Thanks, that's really helpful.
So I'm looking at a direct trade-off between SPF protection and breathability/comfort level. Makes pretty obvious sense. I wonder if a shirt of really thin, loosely woven, fabric sized several sizes too big would work better. I've been told that's the thinking behind Bedouin robes.

For the most part, the trade-off you speak of is correct. However, the commercial UV protection shirts can be quite comfortable and breathable without oversizing. While I had a bit of trouble paying $30 for that white Nike t-shirt, it was one of the best things I brought with me on that trip. In fact, my other shirt that trip, a cheaper UV shirt from Campmor was not as good, got stinky faster and was so thin that mosquitoes could sting through unlike the Nike. Breathability was great. I think the breathability is a function of a layered but loose-knit fabric. On close inspection, it looks like good sized air gaps on one layer offset by the layer beneath so that the air goes through but the sun does not. Weight would please most ultra-light folks.

Down here I tend use a hat with a brim all-around. Not a full size slouch hat. But a light hat with a medium sized brim all-around. I'd like an 'Outdoor Research' type hat or similar but just use an old one i've had for ages.
I also use a first-aid sized 'triangular bandage' (as used for making an arm sling) tucked in the back of the hat and let it run down over my shoulders and pack. If its windy I tuck the tail into my pack to stop it blowing around. The bandage doubles as all the usual bandana functions and its a sling etc if ever needed.

This used with a long sleeve white coolmax shirt of SPF50 rating stops me having problems. It keeps me cool and stops sunburn. The coolmax shirt really works in a dry hot climate, it has a deep front zip to aid further venting if required. I have also used the triangular bandage wrapped right around my face when hiking over long snow patches in sunny weather – it worked well.
A shirt with over-long sleeves will protect the backs of your hands which can really get fried whilst walking all day in the sun.