Sun rash usually shows up one to two hours after sun exposure. The rash can disappear within two to three days, but it may last more than a week. Fortunately, these rashes rarely leave scars.
Common home remedies for treating sun rash include applying a cold compress to the affected areas, using hydrocortisone cream, taking antihistamines to lessen the allergic reaction or taking anti-inflammatory drugs, such as aspirin or ibuprofen, to reduce redness and swelling. If blisters appear, cover them with gauze and avoid popping them [source: Mayo Clinic].
You should see a doctor if your sun rash covers more areas of your body than just the neck, chest, arms or thighs, or if it begins to blister painfully or you start to run a high fever. Also talk to your doctor if you're on any medications that might have caused a photoallergic reaction. Your doctor may prescribe a strong antihistamine or recommend a technique called hardening, which involves gradually increasing your skin's exposure to sunlight [source: American Osteopathic College of Dermatology]. If your doctor offers hardening through phototherapy, he or she will expose you to doses of artificial UV light for a prescribed amount of time.