I have an Ultra Coat covered Waco that's eight seasons old now. It doesn't have a single wrinkle and has never been retouched except to repair damage. The trick on solid surfaces is to rub down a small heated area with a soft cloth. Your iron should only be used as a heat source. I also have a Nosen Citabria that's six or seven seasons old and covered with pre-cut and overlapped sections that shows only the slightest bagging in direct sunlight. probably because of the overlapping I don;t dare heat it as much as I might otherwise.
Walt