Fantastic model by Shadowcraft! This was a blast to build.
For those curious, here were my steps: I printed the model in PLA+ (eSUN) at default scale with 10% infill and supports (needed for several parts of the model). After gluing the two parts together and sanding the seam (I did not get this nearly as good as I wanted it, but I got impatient to finish the project), I sanded the whole model several times, primed (with automotive filler primer) and sanded a couple more times to get it as smooth as possible. This took a long time and I wish I had spent more time on this stage. Next, I primed it with Vallejo's black gloss primer. I then masked off all the parts I wanted to remain black (windows mostly) and put down a base coat of Vallejo model air gold metallic. Then I put down 4 or 5 coats of Tamiya red clear to give it the candy red appearance. I used Vallejo model air black metallic for the brakes and actually kept the brake calipers the original PLA fire engine red color. The wheels were also just kept the original color (eSUN PLA+ black) which turned out to be exactly the look and color I was wanting. I then coated the model with a clear glossy varnish prior to gluing on the wheels. I couldn't come up with an effective way to smooth the rims with sanding, so I actually used an epoxy coating on them prior to priming and painting them which worked surprisingly well (I used Vallejo metal color steel I believe for those).
I made a couple mistakes that I might caution on:
1) I should have painted the black areas an actual black paint instead of relying on the black primer (the primer faded over time and became less glossy, especially after I put the clear gloss coat on it for some reason). I was thinking Tamiya gloss black would be a good option.
2) I thought I had gotten the seam pretty well sanded smooth, but it's clearly still visible once I started putting the clear red coats on. I probably should have looked at it closer when I had the black primer on before proceeding.
3) If I were to do it again I would probably leave out the headlight lenses and just relied on the inside headlight cavities being black instead.
4) I really didn't sand enough on the back of the vehicle, but I wasn't motivated enough to go back and redo the entire base coat by the time I realized how visible the ridges still were.
5) I was originally shooting for a candy / metallic red like the color on the original reveal Roadster, but my color ended up being multiple shades off. I used a gold base, but I probably should have used an aluminum or silver base to give it a redder tone (this almost came out more orange in the sunlight). Also, the model has a pretty nice glitter / sheen to it in person but it doesn't really show up at all in the pictures.
6) Biggest mistake by far was that I forgot to clean out the filter on my spray booth and got a ton of debris on the model during the clear red applications. Some of the dust in the pictures is just from it being outside, but a good portion of it is embedded in the clear-coat. Make sure to do this in a clean area...
Credit to Shadowcraft for making a fantastic model!