I usually wear a jersey made of wicking material such as Coolmax but sometimes wear a wool jersey. The wool doesn't wick and stays wet but surprisingly the problem hasn't been as bad with the wool jersey, maybe because it is thicker so offers more padding between my back and the seat. Or maybe because the wool gets all soggy it ends up clinging to the skin rather than allowing the skin to rub against it.

Every recumbent I've taken on a long ride I've had this day after back breakout problem with. On the Vision seat and Easy Racers Koolback seat there was a large area of pimples going across the top of my back that matched up with the top of the wide mesh seat back. I got a Bacchetta with narrow carbon seat covered with that prickly foam material and the pimples were confined to a narrower area at the top of my back and along where the upper sides of the seat contacted my back. Do you think a Ventisit pad would help?

Interestingly the acne is never comes up after a 50 mile ride but after a 100 mile ride it does. Temperature doesn't seem to have an effect on it, I've had it happen on both cool days and warm days so it seems to be more of a friction problem than a sweat problem.