One Shot makes "poster colors" which go on flat as opposed to their normal "lettering enamel" which is glossy. You could probably reduce it a bunch so it would go on really thin. Might be worth a try. I used to letter race cars and do sign work years ago but I never tried to do the faded old look that seems popular these days. You might wanna use an old fender or something to practice on 'til you figure out how to make it look like you want...

Here's One Shot's site: http://www.1shot.com/home.html