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You have to find out the reason for your rash. Do you develop the rash when you try to remove your back hair or due to using a new cosmetic on your skin? Try and analyse what may have caused the rash. Is it chicken pox rash, poison ivy rash, urticaria or Dermatitis herpetiformis, an intensely itchy skin eruption. Unless I know what it is, it is difficult to suggest home remedies.

Applying aloe vera gel for back rash will be soothing.

I suggest you get it examined from a dermatologist to find out what exactly it is for proper care. Do let me know if you find out so I could suggest specific home remedies for the same.