Guide: Finishing 3D Prints

You’re here for one reason: you want to make your 3D prints look like they weren’t printed at all.

I’ll go over what tools you can utilize for different projects, but not necessarily the only ones available. Whether you just bought your first 3D print or want to get better at finishing your own prints, I’ll show you a few ways to get them ready for painting.

While the techniques and tools here are viable with most filament types, there are some key differences between say PLA, ABS, and PETG:

  • ABS is the easiest to sand and also reacts with acetone, making it an ideal candidate for vapor smoothing and acetone welding parts together.
  • PLA is arguably the easiest filament to print, but also the hardest to sand.
  • PETG is a good medium, although it’s not nearly as easy to sand as ABS.

Most of the tools and techniques listed here can be used with a variety of filaments, even wood.

Acetone smoothing (whether through vapor or brush on) is 100% viable with ABS prints, albeit with some drawbacks. Details may get lost, so it’s mostly optimal for organic shapes. I will not be covering them in this particular guide.

 

CHAPTERS

i. Tools & Materials

ii. Filling Gaps and Seams

iii. Dry Sanding

iv. Wet Sanding

 

If you’d like to help further my endeavors in 3D printing and propmaking , please consider using the Amazon Affiliate links below. They greatly help fund future projects at no additional cost to you – thanks!

 

i. Tools & Materials

In this section I’ll be going over several choices for tools and materials I use throughout the entire process. What works best for me will probably work just as well for you, but in the end it’s your call what to use and when. None of these practices are written in stone – there are an infinite amount of options for each category. This is just what I happen to use because I like to balance between quality and being frugal.

 

 

 

  • Sculpting Tools

These tools have a variety of uses, including but not limited to spreading Bondo and shaping ApoxieSculpt.

  • Sandpaper

Sandpaper can make a print really shine, figuratively and literally. It’s also the most time consuming process of post-production. There’s no shortage of grits numbers and forms available. Knowing what grit and type of sandpaper to use can get tricky. Below are a few examples:

– Sanding Sponge

Sanding sponges, much like sanding blocks, help ensure an even job over large surfaces. Their corners can also be utilized for hard to reach areas. They last quite a long time, even after the original grit number has worn out.

 

– Sandpaper Sheets/Rolls

The elasticity of the backing also makes it ideal for sanding blocks. I cut these into one-inch rectangles to use by hand and also use it to cover used sanding sponges.  Because of their elasticity, they’re great for sanding over organic shapes as well.

– Sanding Block

Attach part of a roll or sheet to the block. Think of it as an interchangable sanding sponge.

 

– Sanding Sticks

These are great for sanding detailed or hard to reach areas. They come in an assortment of grit numbers and are available at any hobby store.

– High Grit Sandpaper

They usually come in rectangular sheets varying from 200-5000 grit. You can cut and apply them to any surface, like a popsicle stick. I generally don’t use above 2000 grit myself.

 

  • Dremel

Dremel is a handheld motorized rotary sander. Usually even the lowest setting/high grit is too much power to for PLA prints, but it works just fine with ABS since it’s heat resistant.

  • Orbital/Triangular Sander

Much like the dremel, orbital, triangular (or any motorized sander for that matter) is way too strong for PLA even on the lowest RPM setting. However, these types of handheld sanders can make ABS prints a breeze to sand broad areas versus doing it by hand. If you work with ABS prints a lot or have a large print to finish, I highly recommend investing in a handheld sander.

 

  • Filler Primer

This is truly a godsend for finishing 3D printed parts – it does a fantastic job of filling in between layer lines making sanding much easier. At least a layer or two of filler primer is a necessity for post-production on 3D printed parts: it helps your paint job stick to the print.

If you’re on a budget, Rustoleum brand filler primer will do the trick. If you can spend an extra $20~ then I would absolutely get 1k primer at the very least. It adheres to the print better, and is just better quality for everything overall. 

 

 

  • XTC 3D / Resin

XTC 3D is a resin designed specifically for 3D prints and nestles in between layer lines. It strengthens a print and adds structural integrity.  If you’re working with PLA this is pretty much a requirement. It’s useful in ABS as well, but it’s not as needed. If you’re going to be working with 3D prints frequently, this item is definitely worth picking up.

 

  • Spot Putty

Spot putty, like Bondo, is generally used for home and automotive repair. It takes only minutes to dry, making it a great candidate for filling in tiny cracks. Generally only ideal for small spots. It’s not quite as durable as Free Form Air or Apoxie Sculpt.

 

  • Free Form Air

Free Form Air (FFA) is an epoxy dough that’s a great alternative to Bondo. It has a soft, almost marshmallow-like texture. This item is somewhat pricey (but it will last awhile), so if you’re not working on multiple prints, Bondo may be the cheaper route to take. Also, Free Form Air is not a good choice if you’re in a rush; it can require up to 1 0-24 hours to cure and be ready to work with, whereas Bondo only needs about 20 minutes.

 

  • Bondo

Bondo is a resin-based filler used in automotive and home repair. While it has its uses in 3D printing, there are pros and cons. Cons: it’s quite toxic so not ideal indoors unless you have taken proper safety precautions. It can also be hard to work with since post-production takes longer than other putties. However its cheaper than most of the alternatives and does the job it needs to. When using Bondo (or another resin-based product), make sure your workspace is properly ventilated (preferably outdoors) and follow the safety documents given with the product closely.

Tip: You can add acetone to Bondo to thin it to use as a brushable filler.

 

 

 

ii. Filling Gaps and Seams

At this point your parts should have already been assembled. If you’re looking for help on how to put together 3D parts, check out my guide on ‘Assembling 3D Prints’ 

You might have a part that looks like this. Or just want to fix a blemish. I’ll show you different ways to fix it!

 

Depending on the 3D print you’re finishing, sometimes you’ll have gaps and seams to fill up between separate parts.

You have several choices, (but definitely not limited to):

1) Weld the pieces together with spare filament.

and/or

2) Use fillers to plug in the gap (although larger prints will still be prone to stress fractures)

 

There are obviously more options, but these are the main 2 that I use and usually together.

 

Welding (Advanced)

How does welding work? And why do you need to do it?

Welding (at least in the context of 3D printing) works by creating a recess, then filling it with spare filament to create a full plastic bond/seal.

When you have a large print consisting of multiple parts, your print is prone to stress fractures and even breakage if the seams are not sealed properly!

 

Welding requires safety equiment – please make sure to use a respirator and gloves!

What  you’ll need:

  • Soldering iron and attachments (or something to melt the plastic)
  • Extra filament
  • Respirator
  • Gloves

 

Want your large prop to last? Don’t skip this step!

It’s a finicky process, but can be made easier with a dedicated soldering iron. If you’re going to be working with 3D printed parts more than once, I highly recommend getting one.

Warm up the soldering blade (make sure to have the proper attachment – it makes a world of difference!) and then make a V shape between the 2 parts.

 

Now there’s a “trench” that you can fill in with the warmed up filament.

 

Use the flat head attachment to melt the filament into the recesesed area.

 

It will definitely look ugly, but don’t worry! Below is a before and after sanding with an orbital sander, we’ll get to that in the next chapter.

 

 

Using Fillers

 

Whether you’re trying to fill in a seam, or just fixing some blemishes, fillers come in handy for a variety of projects.

Fillers come in many materials, colors, and finishes.

As an all-purpose filler, I cannot recommend 3M green Acrylic Putty enough. It dries within minutes and is a fairly durable filler for being a “soft” filler.

Fillers come in hard and soft varities and knowing which to use when mostly comes with experience. For now, let’s stick with one of each. As an example I’ll be using Free Form Air (hard) and green Acrylic Putty (soft).

 

What  you’ll need:

  • Filler of choice
  • Sculpting Tools
  • Respirator  (some are not as toxic, read the directions on the label!)
  • Gloves

 

Free Form Air (FFA) is a hard filler from Smooth-On. Mix equal Parts A and B and mix thoroughly (it’ll turn an even gray color) and it’s ready to go!

 

Apply to the desired areas. In this particular case I’m using it to fill seams and fix parts where the supports were rough.

 

Sculpting tools and palette knives can help clean out some filler in a tight spot.

 

You can also use it to fix blemished areas. While this could easily be fixed with some settings in the slicer software, why waste extra plastic to reprint it?

Mix together and apply liberally.

 

Once it dries it’s completely sandable. FFA takes 24 hours to fully cure before being sandable. Make sure to read the directions on whatever filler you pick.

 

 

 

Filling in Layer Lines with XTC 3D

XTC 3D is a resin specifically designed for 3D prints that nestles in between layer lines, minimizing the amount of sanding that needs to be done. If you’re working with PLA prints, I consider this an ABSOLUTE must.

Note: XTC 3D will smooth out your print quite a bit, so it’s not a good idea for hard edges (like mechanical parts) or incredibly detailed areas.

 

What  you’ll need:

  • Something to mix with, like a popsicle stick
  • Foam brush
  • Medicine cup (or something you can measure weight/volume with)

 

Tip: XTC 3D has a working time of roughly 5-10 minutes depending on how much you mixed. If it starts feeling warm, that means you have less than 1 minute before it’s too hard to brush on. It’s better to work with small quantities and have to mix more, than wasting a large amount at once.

 

If you used fillers, make sure to sand down any bumps first! Do some preliminary sanding with 220~ grit to remove excess filler before applying XTC 3D.

 

Mix two Parts A with one Parts B. Mix thoroughly.

 

A thin coat goes a long way! It’s better to layer on lightly if you need more rather than globbing it on.

You can cover large prints with only 10 ml. For detailed areas, use a thinner coat (or not at all) to avoid clogging up and losing detail.

 

Let that cure for at least 12-24 hours (depending on how thickly it was applied) before beginning the sanding phase. Even though it won’t melt your skin off or anything, it’s a good idea to leave it alone as it’s pretty sticky and irritating to the skin. Just have a gameplan on how and where it’s going to cure.

Seriously – no touching!

 

Regardless of what filler or resin you used, it won’t look quite right yet. We’ll fix that in the sanding stage.

Now that the gaps and seams are filled, let’s move onto the most important part: sanding.

 

iii. Dry Sanding

Sanding is a crucial step for finishing prints.

The goal of the dry sanding phase is to get all the seams flush with each other, as well as removing all layer lines.

 

What  you’ll need:

  • Sandpaper/sponge/block grits from 100 to 400
  • Respirator
  • Gloves
  • Orbital Sander (optional, but highly recommended)

 

First step is sanding off the excess filler (if you didn’t do this before applying XTC 3D) and making assembled parts flush with each other.

This is the time to bust out the orbital sander! Your wrists will thank you later. Although keep in mind any motorized sander is usually too much for PLA prints.

A sanding block or sponge will do the trick. Start with a 60-220 grit depending on how much filler you used and the print layer lines. I personally start with around 100 to 220 because ABS is easier to sand.

 

Now it’s time to sand the rest with 100 or so grit sandpaper. If you’re sanding a thin part that could potentially break, use a higher grit number.

 

There are a variety of sandpaper types and ways you can use sandpaper, for example sanding sticks for are amazing for tight corners and detailed areas.

 

I also use a popsicle stick to stick some sandpaper sheets to help you get some hard to reach areas.

 

Spraying with Filler Primer

Once the first round of dry sanding is complete, it’s ready to be sprayed with filler primer. Keep in mind that you’ll be spraying with primer several times over the course of dry and wet sanding. The process is rarely linear.

 

What  you’ll need:

  • Filler Primer  (rattle can, airbrush, or spraygun)
  • Drying Rack
  • Respirator
  • Gloves

 

Have a plan for how the print will be held up during  spraying. Some people 3D model room for a rod so they can hold it while spraying, I’ve started putting in screws where the prop will be covered later anyway for easy access. The idea being able to reach all angles per coating session while not touching any painted surfaces.

Please remember:

  1. Don’t spray indoors (unless you have a heavy duty spray booth)
  2. Wear protective gear eg respirator, gloves, etc.
  3. Don’t spray too close to the print (about 10 inches/25 cm away is ideal)

 

I personally use 1k filler primer. If you can afford to do so, the quality between 1k and 2k primer versus the Rustoleum type fillers is night and day. However, if you’re on a budget any regular filler primer from any home improvement will get the job done.

Once it has fully dried, resume sanding. There’s really no secret to it – it’s the same repetitive motion until it’s as smooth (or rough) as you want it.

Spray again when layer lines are starting to show again and you’ve sanded down the filler primer away.

Repeat as necessary until the surface is fairly smooth and the seams and imperfections are gone. Around 220~ grit or so, it’s time to move on to wetsanding.

If you see layer lines, you’re not ready for the next step!

 

iv. Wetsanding

Wetsanding is the process of sanding under water. This process effectively eliminates buildup while sanding and helps bring your prop to a glorious, even surface. Are you going for a polished look? More weathered, worn and torn style? These are questions you should have answers to before starting.

 

What  you’ll need:

  • A sink or bowl of water (must be able to submerge the prop at least partially )
  • Gloves

 

To start, get a bowl or filling up the sink with water. I’ve even used a spray bottle in a pinch.

Start with 220-40o grit sandpaper depending on how detailed the piece is and how much it’s been dry sanded.

How high of grits should you work up to? Again, this all depends on your goals. If you’re going for a mirror polish, you’ll need to work up to at LEAST 2,000 grit, then you’ll have to polish by hand or with a buffing wheel with a polishing compound.

When the filler primer starts to significantely fade and reveal more layer lines, it’s time to spray with primer again. Refer to the previous section if needed. It’s okay to go back to the dry sanding phase if necessary, I often do.

Start back up again roughly around where you were before.

My average project ends with 800-1,200 grit and then some polishing compound, and elbow grease.

Now that your print is all shiny (or rough if that’s what you’re going for) coat with a final coat of primer and let dry – then it’s ready for painting or molding! Need help with painting? I highly suggest checking out Volpin Props eBook

 

Have any questions? Get in touch with me on social media!

 

 

 

 

Many thanks to the following artists as this guide would not exist without the knowledge they share:

@Shellshockedprops

@fluxtidedesigns

PunishedProps

 

 

One Reply to “Guide: Finishing 3D Prints”

  1. […] I sanded the 3D print, shot it with primer, and polished it to get it ready for molding. If you’d like to know more about my process, check out my guide on ‘Finishing 3D Prints’ […]

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