Customer stated that AC was not cooling properly. Found system low on charge. Was unable to find a leak, although due to rust buildup on evaporater coil I advised customer that a potential microleak may exist. Discussed options with customer. As per customer, I added stop leak and balanced refrigerant. Cycled system in cooling. Everything is operating normally at this time. Customer is aware of no guarantee on stop leak. Advised customer that we will need to do a more thorough leak check if issue persists. I applied customer coupon for repair.