Martin's suggestion is best if you don't have a clear coat. I have one PSC Pullman with clear coat, and nothing will touch it - not even good grade paint remover.
But here's an interim thing to try: Barkeepers Friend, right next to the Bab-O on the grocery store shelf. Make a poultice of that and water. A toothbrush will help.
I use it on my scratchbuilds, and afterwards often paint. So far no problems. I do a very light bead blast when I can, but trying that on an import can be hazardous due to warping.
