Last week I went thru a construction area where concrete was being sawed. I had no choice but to go thru the run off which was water and concrete dust, leaving a gray film all down one side of my truck and my boat. I was able to get it off the slick surfaces of the truck and the boat no problem. The trailer is a different story. The flat coating seems to have sucked the stuff up. I have tried a brush and a scrubbing sponge with some car wash detergent which gets some of it off but there is still some residue. Any suggestions on how to clean the trailer?