If you’ve been following along in this series of articles (How to Bring Video Game Characters to Life and How To Smooth & Finish Your PLA Prints - Part 1), you have seen some techniques to smooth out and remove the striations you get from a 3D printing. But what if your original model is significantly more polygonal and faceted than the intended shape? If you have a print that looks like this Soldier 76 mask, there are two different ways to solve this problem: using Bondo body filler after printing, or before printing by modifying the model with a modeling software of your choice. In this article, we will be exploring the former.

Even with the roughest model, you can make some incredible finished prints.

Without any elbow grease.


  • Bondo Body filler with included hardener
  • Personal Protective Equipment: gloves, respirator, dust mask

  • Mixing Palette: leftover silicone from molding, pieces of cardboard, 

  • Spreaders: either actual bondo spreaders, strips of plastic scrap, or thin scrap prints

  • Sandpaper: 120, 220, 400, 800, wax polish, and wax sponge

  • Primer and Paint

  • Optional: Dremel and bits, xacto knife

Step 1: Safety First

First things first, put on your rubber gloves and your respirator. To some, the smell may not be too bad, but after a series of intense headaches after mixing some small amounts of Bondo, I highly recommend reconsidering and putting on your PPE.

Step 2: Divide and Conquer

Start by picking one section of your 3D print and focusing on that section with each round of Bondo. In this case, I’ve focused on rounding out the end of the hoses on the mask. By focusing on one section at a time, it is much easier to see the various edges of the print that could easily get lost by adding too much Bondo at a time. With more skill, you can apply Bondo to one surface and if time allows apply to another surface further away on the print, this will help speed up the process but you run the risk of the Bondo drying as you spread it, leaving you with a clumpy mess.

Perfectly mixed and past its prime.

Step 3: Mix it Up

Gather your palette and your spreaders of various sizes, to mix up small batches of Bondo at a time. You’ll want to start with small batches because the Bondo starts drying, or curing, as soon as you pour out the hardener. You should use one golf ball sized lump of Bondo with one inch of hardener. Weird mix ratio, I know, but it’s what is recommended and what works the best. The amount of hardener you should use partially depends on how hot or cold it is where you are working; if it’s a hot day, use slightly less hardener, if it’s cold, use slightly more. Mix the two thoroughly until one consistent color; it should be sort of a peachy-tan. If it’s terracotta red, you used too much hardener and it’ll cure within five minutes, if it’s still grayish it might never cure.

Top Left: about a golf ball sized glob. Top Right: Squirting on an inch of hardener. Bottom: Before mixing.

Step 4: Apply Directly to the Print

Here’s the part that will take the most skill and finesse. You need to find the right balance of pressure to apply to your 3D print. Too much pressure and you’ll just be squeegeeing Bondo across your print and not actually building anything up, and too little and you’ll end with a big swirly goopy mess. You want to very carefully control where you spread it out. While you can always sand away where you added too much, you don’t want to make an already lengthy process even harder on you. Here I feather the Bondo across the surface of the (section) to build up between each of the peaks.

Left: rough and polygonal. Right: smooth and round.

Step 5: Sanding: “It's coarse and rough and irritating and it gets everywhere.”

While the last step took skill and finesse, this step will take time and patience. Start with a very low grit sandpaper, 120 is where I like to start, and go to town on your print. Bondo is basically a resin and talcum powder, so sanding it away is easy, but it will make the biggest mess that isn’t really avoidable. Use small strips of sandpaper and throw them out when they get too gunked up; you’ll actually start polishing the Bondo if it’s completely clogged. If you’re trying to make a large curve like the mouth, be gentle and careful not to sand all the way through to the 3D print underneath. It sands differently from Bondo and won’t wear away as quickly as the surrounding Bondo, leaving you with small peaks.

Notice the peaks between the patches of Bondo, and the beginnings of things smoothing over.

Step 6: Ad Nauseum

Once you’ve gone through one pass of applying Bondo and sanding, you’ll notice some spots that need to be built up a lot or a little, so get back to it. Repeat steps 2-5 until you’ve smoothed a section to your liking. In the case of the ears, they started as a polyhedron, but are now basically hemispheres, this is what you will have to do continuously until it is finished.

Step 7: Optional: Detailing

Bondo is normally used in a body shop for cars, right? Well, same rules apply here, when you need detail shaped into the body filler, it’s best to break out something with more control than simple sandpaper. Using a sharp razor or Exacto knife, you can carve precise lines into your object, in this case I want some small dimples in the cheeks.

Some dimply cheeks.

For the dimples, I sketched in pencil first, then went back over the line with my razor. Then I could gently whittle away the area within the line to create a dimple. And once I had the basic shape of the dimple, I could go back over it with sandpaper to smooth out the straight edges left behind from the knife and to blend the new shape in.

Carefully and gently score and carve in the fine detail

Step 8: Paint and Polish

Congratulations! The hard part is over. Now all that’s left to sanding finer and finer until there’s a nice polish. I started by spraying a primer coat to bring this helmet into one consistent color, even though it wouldn’t last long. Once I had let the paint sit for a couple hours before I got back to it, I started by sanding with a 200 grit sandpaper very briefly, moving up to 400, and finally up to 800. Once it was squeaky smooth, it was time to ruin it once again: spray on your choice of gloss to really make it shine. In this case Rustoleum Gloss Seaside is very similar to MatterHackers Blue. Start sanding again, but this time use water and 800 grit to really smooth the gloss coat. This will bring down the “orange peel” that can happen with gloss spray paints. Once satisfied with that, using car wax, polish your print, and do this several times until you it’s so smooth it’s slippery, and you drop the print and break it (seriously don’t drop it, that is not a fun time).


Left side: Freshly primered. Right side: several passes with progressively gentler sandpaper.

Step 9: The Hardest Step

The final step is to take a step back, look at your handiwork, and call it. It took a lot of work, a lot of sweat, and a lot of dust, but in the end you go from something like this:

The finished product, ready for molding.

And coming next month: how I outfit the entire office with masks. After all, we’re all soldiers now.