Printing through SLA-technique (in stead of FDM) would result in a very smooth surface. SLA pieces can always be sanded down if required, so even high gloss finish is achievable. Unfortunately, for these types of applications, SLA parts (certainly if based on normal CAD drawings as used in car design) would be too weak to support the pressure (air friction) on the parts while driving. And any parts with manually increased thickness would become impossible to print (and too expensive anyway). They could also easily distort overtime when exposed to higher temperatures.
The solution to all of these issues is to reinforce the SLA parts with fiberglass to make them much stronger. It's been done before, even on whole cars (during design phase)!

Unfortunately, there's not that many service providers who have large SLA printers where you could order these as a whole. There's one though in Belgium who can print volumes up to 2 x 1 x 0.5 meter (and has a whole series of these machines), but you'll need to be prepared to pay 10.000-15.000 Euro for such a part (bumper, dashboard, set of 2 doors, ...). You'll probably need quite some very high end cars / tuning lined up to make this kind of investment worthwhile...

You could also use the SLA part to make silicone moulds. I'd doubt though that you would be able to decently fill the mould and get usable parts out of it...

Most small series bumpers are produced with RIM (reaction injection moulding resin, poured into milled moulds). So milling is probably a good first step to investigate... (but it won't be cheap either)