Gather Your Supplies
First things first — Let's look at the materials and tools you'll need to properly prep, sand, fill, and prime your parts. Keep in mind not all of these will be necessary, but having them all will make your life a bit easier and the end result a bit cleaner.
Grab the following from Walmart, Home Depot, Amazon, etc:
- Rustoleum 2-in-1 Filler Primer
- DAP Plastic Wood Filler
- 150-grit Sandpaper
- 220-grit Sandpaper
I also recommend the following:
- Compressed Air
- Bondo Spot Putty
- 400-grit Sandpaper
- 600-grit Sandpaper
As a result of the printing process, each piece will have visible layer lines and possibly a few small imperfections. The end goal is to hide these entirely so that your piece looks like one smooth, clean surface, leaving you with the perfect finish for your final paint and details.
Start off by tearing off a ~3x3" piece of 150-grit Sandpaper and begin firmly sanding your bare print. You will see as you sand the print becomes kind of fuzzy and turns slightly white, this is what you want, it is chipping away at the peaks of the layer lines. Don't worry about getting things too smooth, just spend about 5 minutes on it and then move on.
Now that you've put a dent in those layer lines it's time to start using some primer to help fill in the valleys between layers.
Grab your Filler Primer and give it a good shake to evenly distribute the contents, also make sure you're doing this outdoors and in an environment that's ideally above 70 degrees fahrenheit. You want to spray a light coat on the whole part, making sure to get good coverage on all angles. Be careful with small details because this primer is a high-build formula and if care isn't taken, it can muck up / fill some fine details on the part.
Once you've got a pretty good light coat, wait for everything to dry completely (about 1-2 hours). Making sure it's fully dry will make it a lot easier to sand and keep your sandpaper from getting immediately gummed up.
Spray and Repeat
This next part is going to be the most repetitive & time consuming, but also the most important. Tear off a ~3x3" piece of your 150-grit Sandpaper and start sanding your entire piece, using medium pressure — about the same pressure you'd use to scrub a really dirty dish with a sponge. When your sandpaper gets too gunked up just tear off a new piece and keep at it. You'll be sanding until you can feel the surface has become noticeably smoother and you can see the plastic layer lines peeking through the primer like in the picture above.
Once you're done, wipe down the part and grab your Filler Primer, give it another vigorous shake, and hit it with another medium-light coat. You want just enough so that you don't see much of the print's original color anymore, but not so much that it'll be too hard to sand through once it dries. Go grab something to eat or watch some Netflix for an hour or two while it dries fully.
At this point you've done the first of several rounds of priming, drying, and sanding that make up the main finishing process. From now on you can move to 200-grit Sandpaper and just keep on doing what you just did:
- Sand until uniformly smooth and some plastic is showing
- Wipe the part down, then coat with more Filler Primer
- Wait for it to dry completely
You may notice some spots are being stubborn, or are particularly difficult to hide the layers. Enter the Plastic Wood Filler. After you've finished another round of sanding, you can wipe down your part and apply small amounts of this filler by hand to any areas that need a little extra help getting smoothed out. Just smear on enough to help fill any gaps or rough spots — like I did in this picture — and let it dry about 20-45 minutes before sanding.
If you have Bondo Spot Putty, it can be used just like the Plastic Wood Filler but with much more precision and to fill much smaller cracks, holes, and imperfections. I like to use the filler for large areas, and the putty for small, rough patches or just to help finish off a small area that still has layer lines. Be sure to wear gloves with this stuff!
After sanding, you can hop right back into the cycle by applying another medium-light coat of primer.