I posted a query on this. People were just like, oh use whatever. Well, I'm not convinced that 'whatever' is good enough. For instance, to make a paint flat, satin, or semigloss (I don't think glossy would look good on a bumper) flattening agents can be added, however these are said to reduce the life of the paint. Like they make the paint porous, and then you can't wax it, and if you do you'll regret it, and when you wash it its going to look splotchy, and when bugs hit it, their oils will make bug splotches, and also paints are such high solids these days that you have to add a lot of flattner, and that will degrade the paint, et.

However there are paints that are made to be what they are, without adding flattening agents while mixing the paint. One such paint is SEM Hot Rod Black. I've been told that it won't look splotchy. It may not be as flat as some of the Hot Rod crowd prefers, but its a lot better than putting police car black on a german bumper. There's also Dupont's Hot Rod Flatz, but I've heard mixed reports on this, and there are other Hot Rod Blacks, some too flat for my taste, and others with a nice rich look that still isn't high gloss. One problem though, is that in California as of the end of the year, only water based paints will be allowed, and currently Hot Rod Black et al can only be used if manufactured prior to July of this year.