I've had decent luck with a light duty scotch brite and water. The kitchen kind with soap built in. When it is done you will need to rubbing compound it to get gloss back, but it worked quite well on a couple-layered Giant I had. Lacquer thinner is a no-no on old finishes as many of them are succeptible to it. Modern finishes, particularly urethane are a lot harder to damage by it. Benzene/gasoline can work as well. I used to like using aviation gas(different from car gas), but that evaporates a lot cleaner than car gas.