What is a good first aid treatment for skin rash?

Hydrocortizone. Sorry, I don't know how to spell it. It cools off your skin and it wont itch, and it will go away in a few days .

Area should be clean (use very mild soap or just water) and dry, application of calamine based lotion/creme Atopic dermatitis or Eczema is treated with Hydrocortisone.