I think this is exactly the problem that I am facing. But one side of my trailer is covered with literally thousands of these little black spots, so working on each one individually would take way too much time. This is a two-year-old trailer, but I have only had it a few weeks. So I don't really know how long the spots have been on it, but I am thinking at least a year. Most of them are no longer soft and easily removed with a fingernail. They are hard and tightly adhered. They are not tar, because I have tried a variety of solvents and nothing removes or even softens them. Degreasers, soaps, and other cleaning agents don't touch them either. So far the only thing that removes them is a lot of rubbing with Meguiar's 91 compound, which is way too much work if done by hand. I'm thinking I have no choice but to resort to a power buffer, but I'm still worried that I will damage the finish on the trailer. I will say that rubbing with the 91 compound by hand leaves a nice, shiny finish and doesn't seem to damage the surface at all, so perhaps my fears are unfounded.
I'm grateful for all the advice. As a new Oliver owner it is wonderful to find so much help available on this forum.