Choice of Material
PLA is an obvious choice, but it has drawbacks compared with ABS.
- PLA is more brittle than ABS.
- PLA softens at a lower temperature than ABS.
- PLA is not treatable with acetone for vapor smoothing.
- PLA can not be glued
with (most) solvent-based adhesives.
I would consider ABS or ASA to be good choices for an RC-car body.
Getting Smooth Surfaces
To get a really smooth surface, after printing with thin layers and good print settings to minimize strings and blobs, you will want to treat the surface. The two most common techniques are sanding and vapor smoothing.
Wet Sanding is Great. Dry Sanding is Not.
Sanding is a great smoothing technique for PLA and even for ABS, but one must be careful. In addition to some of the sand paper grains being aligned as proper cutters to remove material, many grains are uselessly aligned and simply contribute to friction. The plastics used in FFF printing are, by definition, thermoplastics and will melt. One can easily soften and even melt plastic with dry sanding.
When the plastic softens, it can form little balls which dig into the surface, or stick to the surface. These hurt the surface finish like snowmen mar a field of freshly fallen snow.
Any sanding should be done wet, with wet-rated paper. Work up through the grits. Lower grit abrasive removes more material so that the surface can be "even". Lower grit lets you sand out the layer lines quickly.
Use a Sequence of Grits
Higher grit papers remove the scratch marks of the previous grit.
Generally, I go up by about 50% of grit number at each change.
When I am removing a lot of plastic and want to end with a glossy surface, I go through this sequence: 36->80->120->180->220->330->400->500->1000->1500->2500->3600->5000. Yes, I know that there are some big jumps there, but I haven't happened to stock papers at intermediate points, and it works for me. Depending on how smooth the surface already is, or how much material I must remove, I start further up the progressing. For 3D printed objects, I start at 220. The last three are critical.
Consider Adding Vapor Finishing to you Toolbox
Vapor finishing of ABS can give some very fine and glossy results. It is worth trying, although be aware that acetone mist is very flammable. If you are heating the acetone, treat it as life-threatening and possibly explosive.
If you use vapor finishing, consider that the acetone goes into the ABS. A warm soak or vacuum degassing could help remove it. You don't want the hood of your RC car to start bubbling in the sun as the acetone is motivated to escape.