Now that it is hot & sweaty and even when not hot but windy I get a wet under arm. I can get a sore under arm after paddling few hours like last night. Mostly one arm and there was no wind to blow any water on it.

Suggestions?

I use a NRS nylon spandex shirt. It says 90% of each.LOL I guess 10% Spandex. I never have a problem when working out side laying brick or block which is a lot of arm movement.

thanks!

I've found two things help:
Bodyglide which is like silicone underarm deodorant. I used to use any chafe prone areas. I used to use tons for triathlons.
The other thing that helps is focus on ROTATING your core. There's not as much movement between your arm and your body if you get most of your paddle movement from rotation instead of arm movement. The paddle also sweeps away from the boat during rotation which helps. I've noticed as my forward stroke has improved I get less underarm chafe.
http://www.canoekayak.com/skills/virtual-coach-power-paddling/

I'll try body glide. Rotate decent funny it was one arm way more than the other. I'll have to be very conscious of my movements. thanks!