What is good to use on an allergic rash from betadine contact?

Wash-off first, then. Like most other cause of contact-allergies, avoidance is best treatment. If +contact, best is to wash-off as well as you can with warm water and soap. Then depending how severe, topicals such as calamide/cortisone can be used if skin is intact/or only minor abrasion. Oral benadryl/zyrtec help with itching. If more severe, you may need eval and prescriptions from your doc. Good luck.