You can not rub out an enamel paint job. It never gets hard enough to do that. When you apply an enamel, what you see after 1 hour is what you got forever. If you try to sand it or rub it out, it will go a uniform dull finish, and never ever shine again. If you want to do a dumb thing like painting a car with a brush, use enamel paint and don't thin it.. If you can do it safely without setting the paint on fire, (seriously), heat the paint up to about 100 degrees Fahrenreit, and brush it on while its hot. This will help a lot to eliminate brush marks. There was a time many years ago when you could brush paint a car with nitrocellulose laquer, and sand it out and buff it to a gloss finish afterwards. This paint was harder than Hell when dry, which is what allowed people in the model T era to do this. You can't even buy that kind of paint now.