I get wetsuit rash behind the knees if I stay out in the water more than 3 hours or hit the water for a few hours several days in a row. I have stumbled upon a preventative measure to eliminate the knee rash.
I had some thermal fleece spandex cycling knee warmers from my old bike riding days. They have a soft, fuzzy interior and Lycra exterior. Here is a link to the ones I have that work:
Testing has shown them to work even if you already have a rash. I developed a serious case of knee rash the other day and was going to stay out of the water to let it heal. Then I recalled that I owned a pair of knee warmers so I dug them out of the drawer and put them on. It looked promising so I went for it and rode 4 hours with no problems. The existing rash was not irritated and I felt no further rash developing. The warmers have a single flatlock seam that normally is worn behind the knee, so I simply put the seam to the front/side so it wouldn't cause any problems.
Please share this solution with your friends, it may help a lot of people enjoy spending more time on their mats if they have to wear wetsuits.