I used to do most of my priming in full sun, still do sometimes. I painted a 28' aluminum camper entirely outside so it was always full sun.
Like the others have said, if the metal gets too hot it can cause issues. I got a lot of practice in that these last two years and have found that if the metal is too hot to hold your hand on, it's too hot to paint. I was spraying epoxy on the roof of that camper, wear only a fresh pair of clean socks standing on the roof, and it was so hot it was burning my feet. Couldn't crouch because it would burn my knees. You'll get a lot of solvent pop and it will look like somebody came and threw sand all over it.
I learned I didn't want to do that on my cars (it was a friend's camper and he didn't care, lol). If I'm going to be spraying something in the driveway (because I'm a front yard hack) I clean it in the shop and then push it out in the drive way right before I spray it to prevent the metal from sitting in the sun and getting hot. Then I'll usually push it back in the shop after spraying but that's because I don't want to speed up my 7 day window on the epoxy.