The standard treatment in many nursing homes is
zinc oxide/A & D ointment mixed 50:50

To clear it up at home, clean the area and dry it well (a hair dryer on low is a good way to dry), and
apply A & D ointment. This often clears it up quickly.

If you have a secondary infection present it could be either bacterial or yeast or both.

If A & D does not work, then do triple antibiotic ointment for 3 days, then Lamisil cream, and alternate.

A serious "butt" rash can be complex in other words.

Once you get it healed up, using A & D after bathing applied to dry skin, will often prevent further problems.