The white sintered nylon is a bit gritty out of the printer. But they clean up nicely, and the flexible nylon is virtually crash proof. There are smoother materials available. Like stereolithographic resins. Those come out with an amazingly smooth finish. But just like some cast resins, they can be a bit more brittle.
The sintered nylon on the other hand is very lightweight and more flexible, making it ideal for car bodies and chassis. Here's a quick video showing the flexibility of the nylon bodies. They are virtually crash proof:
They just require a couple of coats of primer/filler. I use Rustoleum's rattle can primer filler, two light coats. Sand with fine paper. One or two more light coats with a progressively finer sand between. I usually finish off with a 800-1000 grit wet sand. And the bodies come out glass smooth. Like this 427 Roadster:
First coat & sand:

Additional coat or two before final sanding:

Color coat:

Earned her stripes:

After clear coat:
