There was just a post on this topic within the last few days.... you might want to do a search on Marykate on/off or slimy grimy. On/off works the best but is toxic and will eat away the rubber washers in sprayers. I use slimy grimy first using a 2 gallon weed sprayer. If any tough spots remain, I use the on/off using a 2' long brush and bucket. Wash the on/off away using lots of water and watch your trailer. For really stubborn little spots and minor abrasions I use a rag with Collinite's No. 920 Fiberglass boat cleaner. It's in a 16oz clear bottle with white liquid inside. It actually takes out abrasions and light scratches from the fiberglass, black rubs or discolorations in the fiberglass. Good luck!!