How to avoid hard rough discolored skin after a scald from very hot water?

Depends. If it just happened, moisten the area with water and shake some table salt onto the burn; brush it off after it dries. Trust me, this works! it snuffs out the pain immediately and prevents blistering. If it happened a while ago, the die is cast - whatever will happen will happen. Your dermatologist can prescribe various treatments for "hard rough discolored skin".