Many vintage trailers have exposed aluminum that has weathered over the years. On the trailer below, I had to polish the windows. The same techniques can be used for the skin.

I found it is easiest to leave the windows attached to the trailer so they will be stabilized when hitting them with the buffer. You can also screw them to a piece of plywood. Aluminum will polish up as shiny as chrome with patience. I use 3M Super Duty Rubbing compound available at autobody supply stores. You can do it by hand but it will take about ten times as long. I have a Craftsman automotive buffer. It's worth getting a better buffer if you're going to use it a lot but Harbour Freight sells good ones for about .

It took 2 passes to get the windows to shine. You simply pour on a small amount of the creamy compound and begin polishing. A buffer runs at about 1800 rpm. You want to be careful not to let the buffer grab the edges so you want to let the pad spin in the direction of the frame and spin off the edge, not against it.
Most of the glass needed replacing so I wasn't concerned about messing them up. To replace the glass, you just disassemble the pieces on the back, insert new glass and reassemble. If the rubber seals are rotted you can remove them and seal the glass with 100% Silicone. Silicone will last for years ensuring a good long seal.