My A event is likely to be HOT. It’s long(30 hours). I’ve seen many brands have ‘Mesh jerseys’:
Anyone have experience with these(or other brands version)? Are they significantly cooler? How are they in regards to getting sun burnt through the mesh? Would defeat the purpose of being cooler if I would then need the slather sun screen all over my torso as it seems to me that sunscreen hinders cooling ability of the sweat.
I haven’t noticed any sunburn issues personally when wearing radiator fabric shirts. Noticeably more comfortable and cool for me though.
I just got two of the Pearl mesh jerseys, but have not worn them yet. I will reply once I get a ride on them in the next week or two. I have a concern related to sunburn too, but the material seems to offer at least some level of coverage, on par with some of my other “airy” jerseys.
I use the pactimo one and never had any issue with sunburn. I usually just do a spray of sunscreen before hand
I have a Velocio mesh jersey and it is by far my favorite for Tucson’s summer (read: hot). No sunburn issues and I do not put sunscreen on under the jersey.
Ive no experience of those brands but I picked up one of these which I think is similar Castelli Climbers 3.0 SL Short Sleeve Cycling Jersey - SS21 | Merlin Cycles and for my upper body shape it has been ideal in the heat of Tenerife and Southern Spain and its been OK for the SE England summer too. Edit and my chest/back/shoulders are still milkbottle white 
No problem with sun burn there despite not using suncream under the jersey.
Velocio Radiator mesh jersey user here. Very very comfortable, and have not had any issues with sunburn in 95F+ riding in the sun. It is definitely cooler than other non mesh jerseys. Recently, purchase Pactimo mesh, haven’t tried it yet.
Velocio Radiator and Concept Radiator are fantastic, both use Polartec Delta fabric. They are particularly good when it is humid. I’m sure they would be great in drier environments as well.
I have the Le Col Pro Air (similar thin/mesh jersey) and agree it is fantastic in the heat but getting burned through it is definitely a valid concern without sunscreen. I wore mine on a 5-6 hour ride in Texas July sun and got a really awesome tan/burn pattern on my back. The actual mesh pattern was even burned into many areas.
Exactly what I DON’T want to happen! 
I don’t know if they qualify as “radiator” jerseys, but I’ve used the mesh craft stuff quite a bit in our Texas heat and sun. Definitely cooler than regular jersey and definitely let’s some sun through. I am pretty fair skinned and I don’t typically get a full on sun burn, but a little red and the tan lines from my bib straps are always there.
The ULTIMATE cyclist’s tan!!
Makes the days of mesh glove tans seem quaint!
if you have teenage kids, I highly suggest taking them to the pool ASAP to ensure proper humiliation. 
Here is the reply I got from PI when I asked about the PRO MESH Jersey…
Due to the material and construction this one came back from testing with a very low UPF rating of around 10. We typically would advertise any UPF ratings of 30+ under the features section of the product but with this jersey the mesh doesn’t provide much protection.
I burn. I always wear a Uniqlo airism (Mesh or Micro Mesh - micro is better) base under my bib straps. Doesn’t add any heat, keeps my straps from digging in, and lets the air flow.
I’ve been using Poc and sportful mesh jerseys without issues
I wasn’t impressed with the Velocio jersey for the money.
30 hours seems like a problem. That is probably at least 8-10 hours in the hot sun. You might burn through any jersey without protection, but a mesh one is going to give you the least. I might ask the question, how do folks protect their skin for 30 hour events. I’d be worried about my arms, leg and face for that duration.
actual exposed skin will get sunscreen reapplied. front of the torso should be shaded for the majority of the time. the back is my biggest concern.
The eliel diablo jersey is fantastic, check it out. Mesh up front and more normal jersey material on the back. I have yet to get sun burn on my back—-used diablos all summer last year. The mesh up front feels like your riding without a jersey, its wonderful.
I have the Le Col jersey that wfenwick has and it is noticeably thinner/lighter than the Pactimo ascent aero mesh jersey. I wouldn’t expect that sunburn to happen in the Pactimo…but I’m also not sure the Pactimo is much more airy and light than most normal jerseys.
I use P20 SPF 50 all day sun oil on exposed skin. I wear SPF 50 arm coolers. I wear a cycling cap to shade the face. I will sometimes wear a buff round the neck. Cap, buff, and arm coolers periodically soaked in water if really really hot.