Per a suggestion above, before replacing it, I hit it gently with 120 grit sandpaper -- lots of white dust came off! I kept sanding till it was smooth -- scrubbed it with a rag -- it is now quite acceptable, if not exactly perfect.
After doing some shopping on the net, it turns out that a replacement is just a few dollars. So if periodic sanding is not working out, I can always get a new one.
I like the idea of that plastic paint -- I have never heard of that. Since my poor trailer bakes in the sun every day of the year (literally -- we've had a total of three or four rainy days in the last year), there will come a time that I will need to touch it up, I am sure.
