rubber bed mat

My factory rubber mat gets a like mildew on it and smells. It sits outside in a carport. I scrub it and months later it comes back.  any suggestions on how to keep it from coming back. Has anyone else had this happen to them?             

Clean the mat with Borax to disinfect and keep it dry as long as you can to avoid mildew.

Are you coating it with anything? like armor all etc?