If you aren't using a super-absorbent disposable already, you may consider switching at least temporarily until the rash is better. You may need to change the child in the middle of the night as well. If the problem is really bad, change the diaper very, very frequently - every hour or so during the day, and once or twice at night. Next, moist, softened skin is less resistant to the abrasive effects of diapers and wet-wipes. For this reason, you may want to try to do less scrubbing of the skin, namely rinse it off rather than scrub it off.
Severe diarrhea can produce an appearance of the diaper area best described as scalded.
This type of rash is really a chemical burn from the alkali in the stool, and looks it. Treatment of this type of rash involves dryness, of course. I'm in favor as well of restoration as much as possible of the normal acidic pH of the skin. When rinsing the baby's bottom, try putting a little vinegar in the water in the basin. Dip the baby in the warm water, and gently cleanse the bottom, avoiding scrubbing with a cloth.