Like Bob said above, it will depend on the exact materials to some extent. That being said, if I had a flat-sided trailer, I would be looking at some of the marine products on the market. They have some pretty bold claims, and if half of them are even some what true, you could use them to get a nice shine back on your trailer that would be likely to last. If it can handle being exposed to the water day in and day out, it can certainly handle your TT sides...
Just a thought.
2014 Jay Flight 28 BHBE
2015 RAM 2500 6.4L HEMI, Tradesman 4x4, 3.73
Blue Ox SwayPro (BXW 1503)
Upgraded from an REI internal frame backpack and a Eureka 1/2 dome tent!