To prevent adult diaper rash you should change your adult diaper often and apply some diaper rash cream or petroleum jelly to your skin that is underneath the diaper.
Adult diaper rash is caused by urine and poop in the diaper and bacteria and by staying in the diaper too long your skin gets the rash.
So apply some diaper rash cream to your diaper area such as your penis, balls, etc and your butt to help prevent the diaper rash.
Also allow your bottom to air out for 30 minutes to an hour between diaper changes.