Fused-Filament Fabrication (FFF) or Fused-Deposition Modeling (FDM) -"3D Printing"- create amazing parts, but generally with visible layers. There are two main methods for smoothing those layers to obtain an injection-molded plastic look: chemical-smoothing or sanding. Chemical-smoothing is dangerous as it involves toxic and highly flammable chemicals, and sanding takes patience and well-toned hand muscles or skill with a Dremel.

As an alternative to these two methods, I looked to polymer clay as another form of smoothing.

My conclusion:

Applying the clay takes patience and a little skill (with the help of "tools"). Depending on your level of sanding skill, applying the clay can take longer or shorter than sanding. With regards to toxicity, I'd say it's safer than chemical smoothing, but I still suggest using ventilation and a non-food-used oven when baking it.

A major downside is the tendency for the part to shrink during the baking process. This has to be taken into consideration when creating fitted parts. The upside, is that this process can aid in attaching separate parts together permanently and can fix defects that the other two process cannot.

For me, this is my preferred method for smoothing, as I can apply the clay while watching TV and not have to worry about concentrating too hard (whereas with a Dremel, you need to pay close attention to what you are doing). If I get better at sanding, I may decide that applying clay is too long of a process.

Amendment: Because Polymer Clay is a polymer (plastic), it cannot properly harden at a lower baking temperature than given in the instructions. This means that this technique can only be used for ABS, or other higher extrusion temp materials.

Credit:
The files used to illustrate the process can be found here:

Golbat Wall Sconce by WeeksB (me) https://www.thingiverse.com/thing:3381571

Pumpkisaur Pokemon by chicagolandgeeks https://www.thingiverse.com/thing:1845663

Bulbasaur Pumpkin Tealight Remix by Blackhartfilms https://www.thingiverse.com/thing:3169901

Step 1: 3D Print the Model

Print out your desired model. This can be done on your own printer, a friend's printer, or you could use one of the many services to get files printed for you. Assemble the parts, if necessary, using your preferred form of bonding.

Depending on the file, and the skill of the printer operator, you may find defects in your print.

Certain defects and unwanted appearances can be solved by the polymer clay smoothing method:

Layer lines (shown on the printed wing and assembled Golbat)
Sagging (shown on the jack-o-lantern)
Pitting
Seam lines
Assembly gaps
Some incomplete or broken parts (this depends on your skills at molding clay)

Note: Warped, distorted, or severely incomplete parts might be able to be "fixed" with the clay, but will likely require strong clay modeling skills and rely on sections made of solid clay.

Step 2: Apply a Layer of Polymer Modeling Clay

Tools:

Polymer Modeling Clay***
Clay Sculpting Tools

I personally used white Craft smart Polymer Clay and just a metal spoon and a toothpick as my sculpting tools. The spoon helped me smooth and spread out the layer of clay and the toothpick allowed me to get small details, both by putting clay into tight spots and removing excess. With proper sculpting tools, like dental picks, a faster, better job can likely be performed, but are not necessary.

When applying the clay, aim to get only enough clay onto the part to feel smooth to the touch when you brush your fingers over the part. The images are shown with the thin layer of clay they were baked with - you don't need a thick layer. I did not cover the bottom of any my parts with clay nor the inside of the jack-o-lantern so you do not need to completely cover the object in clay, just the areas you want smoothed.

You can also use the clay to build up missing sections, cover unwanted defects like seams or gaps in joined parts, or you add your own creative touches and features to the part. Be sure to remove any excess material that might soften or eliminate features.

Work under the assumption that "what you see is what you get." Meaning, that any non-visible features will remain not-visible, and any rough surfaces (or fingerprints!) will remain after baking. So, get the part as smooth and well defined as you can.

Tip: I found the clay to be rather crumbly at a "room temp" of 60˚F. A little heat, helps softens and makes the clay more malleable.

*** It was pointed out to me by Omnivent, that because Polymer clay is...duh... a polymer (read: plastic), there is no way to lower the baking temperature and still have the polymer properly "harden". What I was seeing, was the clay being dried out. I suspect that this will be an issue in the long-term or with parts that require a certain amount of strength.
Alternative "clay" options to try instead:

Plastic Padding

Air Dry Clay

Epoxy Putty (GreenStuff)

Milliput
I will try out alternatives and get back to you all on what I personally suggest

Step 3: Bake

Tools:

Oven (PLA: <170˚F / 77˚C ~ ABS: <190˚F / 87˚C)***
Baking sheet or aluminum foil

Place your part on a baking sheet/aluminum foil and, if you have multiple parts, spread them out so that they are not touching. Start with 30 minutes and check the parts often. Leave the parts in until they feel slightly hard.

Note: Your parts will shrink slightly, especially if they were printed with a low infill. Adjust accordingly.

Learn from My Mistake: Craft smart recommends baking at 275˚F (135˚C) but I found that this was much too hot for PLA. 275˚F is above the glass temperature and the PLA quickly becomes soft at this temperature. The top of my Golbat (shown) was baked at 275˚F for a fraction of their recommended time (10 minutes), and resulted in very noticeable dips in the surface. I had to re-coat the entire model in clay and re-bake at the lower temperature.

Bake Temps: Bake the parts at no more than 170˚F (77˚C) for PLA-based parts. - If you have an oven that can be set lower, PLA glass temperature is about 140˚F (60˚C) so I recommend trying 135˚F (57˚C). ABS's glass temperature is about 212˚F (100˚C), so 190˚F (87˚C) should work well.

Safety: Polymer clay is stated as being safe, but I noticed a distasteful off-gassing smell when baking. I highly recommend using good ventilation and an oven that is not used to cook food. I also used single-use aluminum foil instead of a reusable cooking sheet for this reason.

*** Because Polymer Clay cannot be properly hardened at a temperature below 275˚F (135˚C), this approach can only work with ABS (or other feedstock material with a higher extrusion temperature - such as PCABS). For PLA, an alternative "clay" must be used (see the endnote in previous step for suggestions).

Step 4: Paint

Tools:

Paints
Paint brushes
Pallet

You might find you like the stone-appearance of the clay if you applied it thickly, but most likely, you wanted to smooth your part so that you could paint it. I've found that the clay works fairly well as a base layer for painting, but if you want to be certain, add a base coat of Primer first. I used acrylic paints very successfully for my models and sealed the paints with a top coat of clear satin spray paint.

Note: The image with the Golbats, shows the difference between a painted part that had been smoothed first (bottom - grey) and a part that was not smoothed first (blue - top)