Whenever my son gets a bad diaper rash (usually because he's been eating some new foods that don't agree with him too well), I bathe him in an oatmeal bath, let his bottom dry out well, and apply a coat of raw honey to the rash. It drastically reduces or eliminates the rash by the next diaper change. Then I'll apply virgin coconut oil at each change until it's gone (it's less sticky). It's never taken more than a few hours or overnight to get rid of any rash, regardless of the severity. The honey as a first step is crucial, so don't skip to the coconut oil.
Raw Honey for diaper rashes: my grandfather used to raise honey bees, and sell the honey.My mother used the raw honey on diaper rashes. It was all that ever touched the bums of my three younger sisters. She had a small container of it in the diaper bag, and some q-tips to apply it. I love telling people about this. they don't believe me, but a few have tried it.