Sun rash is not the technical medical phrase and it can point to a few different rashes. One rash that is referred to as sun rash has to do with sun poisoning. Sometimes it's also called photodermatitis. It can have blisters and be quite red. My friend during my childhood that had red hair and freckles was prone to this problem. Some of the blisters has a watery fluid inside of them. She said she was allergic to the sun and this seems to be the case.
My childhood friend had to wear hats and sunglasses and avoid the sun during peak hours. She also developed skin cancer in a mild form in her thirties due to her sensitivity to the sun. I remember when we were kids that if she was in the sun too long she would coat herself with calamine lotion for relief. Other treatments that are recommended include using oatmeal baths and corn starch. The condition may require antibiotics if the red rash spots open up. You also may get a sun rash poisoning if you take certain medications. Read the warnings on medications as they will indicate if you should avoid the sun. We tend to not read labels, but this is important to prevent a burn.
Another way people use the phrase sun rash is to refer to a sweat rash or heat rash. This results in red pimples or small welts in areas that perspire. It can happen after working out, extensive time in the sun or a propensity to sweat. Technically, this is not what is usually referred to as a sun rash since it usually means photodermatitis, A heat rash will generally resolve in five days and can be treated with oatmeal baths, cool compresses and corn starch. For sun rashes that are due to photodermatitis, be aware that even a few minutes in the sun can result in a skin reaction.