It doesn't last long, but it works nice and easy to apply.... Pledge furniture polish. Spray on, wipe off. Of course, after the second rain it's pretty much gone, but sure makes it shiny, easy to apply and does help. Just not long term.
I traded our suburban for our first duly pick-up truck. Before negotiating a trade in value, I washed the suburban and then gave it a Pledge furniture polish wax shine. The dealer couldn't believe how nice the finish looked! I got top dollar for the trade, and "got out of Dodge" before it rained! So far, on my current trailer, it doesn't seem to hurt the graphics. (almost 4 years now).