The two most common desktop-style 3D printers are FDM (for fused deposition modeling) and SLA (or stereo lithography). FDM printers lay down layer after layer of filament, resulting in objects that have 3D printing's trademark layer lines. SLA printers use resin and light to produce prints that are often without lines and highly detailed. But neither method is particularly adept at producing shiny objects.
That's not to say traditional 3D prints can't result in shiny objects. YouTuber "Uncle Jessy[1]" post-processes large 3D prints to produce cosplay masks that have that Iron Man shine. He uses a combination of chemical treatments, old-fashioned sanding, primer, and painting techniques to get the look he seeks.
Also: ZDNet's DIY-IT 3D Printing and Desktop Fabrication Discovery series[2]
And let's not forget the incredibly ill-advised and dangerous acetone vapor technique[3] used to smooth ABS 3D prints. The idea is you set a small amount of acetone inside a closed vessel along with your print and allow the vapors to dissolve the layer lines. This, of course, is both highly toxic and flammable, but it can be effective if you time everything exactly right.
There are also commercial smoothing devices like the Polymaker Polysher[4], which use aerosolized isopropyl alcohol to dissolve layer lines from the company's custom Polysmooth filament. While somewhat safer than acetone vapor smoothing, the process is still a bit hit or miss in terms of results.
Selective shine
What none of these techniques can produce,