I would actually stray away from acrylic enamel. There about 4 different types of auto paint:
Spray bomb, which is laquer based. It is the thinnest, weakest, least chemical and UV resistant, and has the worst adhesion properties. Colors and primer are very limited, and cans can add up quick. Biggest plus: it's readily available to the general public and easy to use. Can also be touched up in a matter of minutes. No mixing, catalyzing, etc.
Next up is acylic enamel. A step up from anything laquer based, has good adhesion and UV resistance, pretty tough. In terms of primers there are many more choices as well. Downsides are, it takes FOREVER to dry. Even with the proper reducers and catalysts enamel paint could take easily up to a month to fully cure. It is also not truely chemical resistant. Brake cleaner, methylene chloride, anti-freeze, they can all take it off pretty easily.
Moving along we have single stage urethane. Something I have come to love. No need for clear, just primer, shoot, and leave it. Is the most chemical resistant, hardest, fastest cure times, UV stable, best adhesion, and lays down extremely easy. For painting an entire car you might want to use a basecoat/clearcoat system, but for bumpers, fenderwells, racks, sliders, underhood coating, this is all I use. You can also get it super glossy, satin, flat, or anything in between.
And lastly there is your everyday bascoat/clearcoat system. This will by far last the longest, but is a little more work for something like a bumper. Also more maintenance to keep scratch free, but the clearcoat is a little bit harder than single stage urethane without one. But both share the same properties as far as toughness etc.
In your situation, I would use some DTM (direct to metal) epoxy primer from TCPgolbal.com, topped with some Hot Rod Flatz "texture" satin black single stage urethane. Total cost, maybe $75 for materials. Another $40 for the gun, and you will be able to use this stuff for a long long time.