If you're down to the bare metal, the best solution is going to be paint.
Finding an exact match will prove to be difficult. However, here is a suggestion. Pull out the yellow pages and look for Automobile Body Shop supply houses near you. Stop in and talk to the counter guys and explain your situation. Most of the better shops have a "camera" which they can take out to a vehicle (or in your case, the trailer) in the parking lot and capture the color of the paint. Then they can mix an exact match. To apply, most of these places can mix the paint and put it into a common spray can for you as well, so you don't need to worry about a paint gun, compressor, reducers, etc.
To make a very invisible repair, you'd need to clean and prep the surface, then apply the paint to the affected area overlapping the "good" area by a little bit. Once it dries, you would wetsand (using a very very fine grit sandpaper and water) to level the area out and then remove the scratches with a polishing compound.
Hope this helps.