Where is the rash? Is the skin irritated by chaffing or from sweating? if it's from chaffing like on your chest, butt, or the inside of the thigh I would suggest you get better clothing for whatever exercise your struggling with. I've never heard of sweat affecting your skin.
Clothes that dont chafe. You can get cream as well. I use vaseline in on the old nipples when Im wearing my wetsuit with all the sand and everything. Not sure if it will work for sweat rash.
