I’ve been looking to up my 3D print finishing skills. A friend of mine is planning to cosplay as Mei. Project begun.
She said she wanted it life-sized, so I went on-line to see if there were some completed projects. What I found were a bunch of people doing more electronics than I was really looking to do, and some nearly-finished 3D models.
I settled on the design here: https://www.thingiverse.com/thing:2011610
Design bits left to do:
- A face
- Body accents
- Changable eyes (stretch goal)
- Ability to puppeteer ears (stretch goal)
I’m printing at 95% size, due to my build plate being just a tiny bit too small for full size. I have been dutifully informed that this makes me a monster. So be it, but I’m a monster who’s building a robot!
The first issue I ran into was that there’s something wrong with the model, and that Cura made it float above the print bed. When I rotated it, and told it to chop off the blank space, everything seemed fine. Then I tried to print it, and for some reason the darn thing was printing sideways compared to the orientation in Cura. Never seen that before. The solution was pretty simple: I brought the STL file into Blender, reoriented it, applied the rotation transform, and exported it as an OBJ. Cura liked it. The printer liked it. I had to make sure the ENTIRE bed was level, and on the third attempt, we were finally printing.
Fast-forward 50 hours.
The part looks pretty sweet at first glance. Then… There’s a problem., which is much more visible once the support structure is cleared out, and you’re looking at it head-on.
For some reason, there was no support structure for the top of the visor, leaving the printer to bridge it on faith. It eventually got there, but things are kind of messed up. I wanted to improve my finishing skills, and Snowball has apparently accepted the challenge. As a result, today I bought my first can of Bondo.
Sanding Pass #1
I really want to do the finish right on this project, which means really taking my time. The first step was 65 grit sandpaper to remove excess material from some printer passes, as well as to generally rough up the surface so it accepts paint more readily. This is also the time to clean out any printer issues like globs or lips from issues such as the bridging problem above.
When I primed this piece, I used an automotive filler primer. This does a great job of filling in the trough layers. When primed, the layers seem to visually stand out a bit more, but that’s OK, since once things are dried, we’ll smooth it out.
Sanding Pass #2
This time with 100 grit sandpaper, with special attention paid to little burrs and leftovers that I may have missed the first time around. The surface is now much smoother from a visible and tactile standpoint. If I quit here, it’d probably still have one of the nicest finishes of any of my 3D printed projects, but I’m really enjoying the zen of craftsmanship, and will continue to take the appropriate time on this project.
It’s probably strange for someone with as much experience building stuff as I do to admit: This was my first experience with Bondo. I know some people who love the stuff, or will even use Bondo for first-pass filling, instead of the filler primer that I used. Not for me in a million years. The Bondo can is basically a giant warning label with a trademark stuck in there. I’ll use it for filling and repairing, but that’s IT. The can doesn’t even have instructions, but here’s the low down for those who don’t know:
Step 0: Go outside, for the love of your lungs.
Step 1: Personal protective equipment (PPE). Nose/mouth, eyes, hands. Have some paper towels. Have something you don’t like to mix on. Note: I also have a spare glove in case I need to pull of the one I’m using and pop on a clean glove.
Step 2: Open can, try not to retch. Use a disposable stick of some kind to scoop out white Bondo from the can.
Step 3: Put a small line of hardening agent on top of the Bondo. The more yo use, the faster it hardens.
Step 4: Mix Bondo with hardening agent (which thankfully contained a dye so I could tell how well it was mixed
Step 5: Apply to damaged area with stick, or gloved hand. I just used the nitrile glove in this case, careful not to catch it on any sharp bits of plastic. Be very careful not to drop or splash.
Folks say it’s done in 15 minutes, but I kept my project outside for a while so it didn’t stink up the house.
It’s hard to argue with the results though. The damaged area should be workable now. I’m out of daylight, so it’ll have to wait until tomorrow. There’s no way I’m sanding that junk indoors.
The body, hot off the printer
Then primed and sanded with the 65 grit sandpaper.
I noticed a number of hairline layer areas, where the filament was too low, instead of too high. These were tiny, but visible, and I could feel them with my fingernail. I was dreading using Bondo to fill in these tiny imperfections. I was about to accept them for what they were, when I watched the following YouTube video and had a “Why Didn’t I Think Of That” moment: Spackle. It’s cheap, should get into small cracks, and it’s WAY less toxic (it seems the author of the video feels the same way I do about the stuff). Bondo still has it’s place, but that place just got downsized.
This much spackle, however, is overdoing it a bit. Turns out that spackle doesn’t wet-sand well. So here’s how I’ve best learned to use it: Rough sand, wipe clean, spackle, rough (dry) sand briefly, wipe clean, primer. The primer will help keep the spackle from getting too wet, which will in turn help keep it from coming off.
Snowball’s ear flaps have little holes in them, so I used those to align the two halves for gluing. I then used spackle to fill the crack. It did an admirable job, though could have been a little better. I’ll take that over dealing with the awfulness that is Bondo though.
This is spackle, with primer on top. The crack between the halves is still clearly visible, though some of those deeper layer lines turned out to be just as problematic.
The faceplate was missing from the 3D printer files that I downloaded from the Internet, so I had to design my own. Basically it started as a sphere, and then I chopped a whole lot of stuff out of it to get the partial dome shape you see here. I did a test print in white filament before moving on to the much more temperamental PETG, which I made a separate post on (3D Printing Clear PETG). Measure twice, print once… happily the first model fit pretty good. I just then had to figure out how to finish the part nicely.
While I was printing in PETG, I also printed the LED holders for the thrusters. They came out pretty clear. I’m likely going to hit these with a medium grit sandpaper to frost them a bit so they diffuse light better.
Polishing Beyond Reason
Since this is the first project that I’ve really gone to great lengths to get an excellent finish on, I figured it’d be worth my time to see how polished and shiny I could get a 3D printed part. It turns out, you can get parts really shiny.
I knew somewhere in here I was likely wasting my time, since the plan was to spray paint all of the parts. However, I wasn’t sure at what level sanding really started to lose value. What I learned: If you’re spray painting, I can’t see much reason to sand higher than 400 grit.
This thruster was dry sanded at 60 and 150 grit, then wet sanded at 200, 400, 800, 1500, and then 3000 grit (once you have worked your way up to those high grit numbers, each pass doesn’t take that long). After sanding it at 3000 grit, I also used some Brasso metal polish to finish the job. The result is visible below: It felt as smooth as a cue ball, and reflected light fairly well. There were still a couple of pits from 3D printing imperfections, but that didn’t ruin the overall effect. There’s still a lot of filler primer on this part: It’s doing its job! Again, some wasted effort since I covered all of that amazing smooth finish with more primer and paint, but it’s kinda cool to know that you can get a PLA print up to a glossy shine.
Paint Patching Pits
There is a point where you’ll have a great finish on your part, except a couple of nagging pits or extra-deep layer lines. Thinned out spackle doesn’t adhere well enough. I’ve heard tale of folks using baking soda and a drop of superglue to patch that kind of thing (I heard about that after I used this technique). I just decided to use some cheap art paint, and it worked remarkably well.
Find the pits you want to fix. Use a small art brush (like for painting miniatures) to put a thick glob of acrylic pant on it. Use a scraper or the edge of a folded piece of paper to smooth over the paint. Let it dry, repeat if there’s still a pit. After the paint has had plenty of time to dry, sand down the excess, you should see your paint color remaining in all of the small imperfections you’ve just filled.