How can you naturally cure allergic rashes?

Avoid allergems. The only way to cure allergic reactions if possible is to avoid the products or conditions that cause the allergy. Unfortunately that may be difficult because of the chemicals in products and the environment. You may be able to to decrease symptoms amd use less medications.
Avoid the allergen. Stop the exposure to the thing that you're allergic to and it should go away. If not, then see an allergist.