I recommend using Triple Paste brand ointment for diaper rash. This is the best protective and curative solution or answer to diaper rash I've found. When I have a rash starting I can use this to, at least, keep the rash from growing or spreading. It's a good protective. Not only that but it seems to help in healing the rash, too, at least if the rash hasn't gotten too large or "strong."
The best cure for rash is air and hydrogen peroxide, but when I've got to stay in diapers Triple Paste comes to the rescue.