I have a customer who hired a previous contractor to build a screened in porch. This contractor hung Green Board on the exterior ceiling, and then walked away from the job for whatever reason. The customer would like me to finish and texture the drywall. Is this recommended or even possible for and exterior room open to the elements?? I have never done or even attempted to do drywall outside and my understanding has always been it won't work. I'd REALLY appreciate some other opinions on this. I've tried to explain the reasons why i don't think it's a good idea to the customer but he is insistent that i do it. My policy is not to do anything i don't believe i can confidently stand behind so i told him i'd do some homework on it. Any thoughts??