[Tamar]

Scuderia_Turini pid='2628' dateline='1561848956']Nice car Tom,
How do you get such a  nice smooth paint job on the 3D printout ?

Leo

I guess this is my que to join in and add my 2cts.

As part of the SV "development" team, I also recieved one of the prototype bodies. 
Having volunteered to the task of drawing the decals for the Signatec livery I initially had no plans to paint the body.
Mine would just serve as a template to apply the paper test stickers to....but...they just would not hold onto the 'bare" SLS surface.

With Tom already doing surface and paint work with the usual Tamiya cellulose base spray paint rattle cans & copious amounts of elbow grease, I thought it might be a good idea to try a different approach, one that might require less elbow grease and patience.
As I've achieved some very good results with painting 3DP rear wings with Vallejo surface primer I decided to go down the Acrylic route.

One of the main challenges with working with 3DP SLS material is that it is very hard to sand and that it is very poreus.
If you use (thin and fluid) cellulose based primers a lot of the paint gets sucked into the pores of the SLS material. 

Rattle cans are notoriously smelly and produce a lot of overspray, as Tom mentioned you need a lot of layers and sanding in between to build up a smooth surface while trying to keep the fine body details. (which he achieved very well) 

   
Vallejo Acrylic Surface primers are water based, so less smelly and have the advantage that they can be applied with airbrush straight out of the bottle, but can also be applied with a regular brush. Using a regular brush means its much easier to apply thick(er) coats of primer with good control of where you want to apply it..and where not.
Vallejo surface primer also has great self leveling properties, so even when applied over rough surfaces it dries with a very smooth surface finish.
But there's a catch...

Web Myth: You can not sand Vallejo Acrylic surface primer
Search the web for tutorials on Vallejo surface primer and each and every one will tell you that you can't sand (Vallejo) acrylic surface primers. 
Well..that is true...and not true.
It is true for the very thin layers that most modellers use on their static plastic/resin models. They've done all their body preparations, sanding and filling gaps before they apply the surface primer. They'll spray the thinnest layer possible just enough to get a smooth surface for the final colour coats of acrylic paint. Unlike cellulose thinner based primers that etch/bond to the surface material, acrylic primers create a thin film that "clings" to the surface like a wrap.
If you try to sand these thin layers you will cut through them at a certain point and the acrylic primer will loose its ability to cling to the surface and actually start to rub and peel off. 
Bye bye smooth finish.

   
But thicker coats of Vallejo surface primer can be sanded as long as you use fine grit and don't sand it too thin.
Acrylic paints are softer and remain more flexible than Cellulose/thinner based paints, which are handy properties when painting slot cars that have SLS printed bodies.
The image above shows the Surface primer on the un sanded front splitter, where as the nose cone and the front wheel arches have been wet sanded with fine grit. How fine is fine? well at least 800 grit, but I do most of my sanding with >1200 -2000 grit.



Use the sanding sticks for square and hard edges and paper for round and compound curves
An other useful tool to preserve the shapes and details of the SLS body surface while sanding the surface primer are these fine sanding sticks from Hobby Stix and Flexi File. They come in various grit and have a foam core so with a sharp set of pliers you can cut them to shape and size and get into almost every nook and cranny of the bodywork.

   
Semi matte or gloss finish
Above is the body after two-three coats of brushed on regular surface primer which has a nice semi matte finish, perfect tone to replicate the bare carbon parts on a sports car like wing and mirror supports, splitters diffuser etc etc. Off course wet sanding between each layer.

Below is the body with an added layer of the gloss finish surface primer, this coat was applied with an airbrush.

   
So looks nice, but again there's a catch
If you use Vallejo surface primer it means it's safest if you stick with Vallejo Acrylic paint for your final color layers as thinner/ cellulose based paints can 'burn" and "eat" the acrylic primer.
You might get away with it by applying vary thin layers with an airbrush, but I would advice against rattle cans.

With kind regards
Tamar