Mu father had this problem before because he used to fishing very often. My mother bought "Badedas" for him to bath or shower. For many years he didn't have this problem anymore. Some of our friends also tried and the marks on the back all gone.
"Badedas" is a bath gel, get the original one (green), Made in England. You can see in Watson's, but it's very expensive there ($140,00) if you buy from the drugs store, it would cost around $70 - 80,00. It smells very good. Even my baby loves taking bath with badedas because it makes bubbles. Only few drops is enough for you to take a shower. In summer, you feel so relax after using it.