Introduction to Finishing PLA Prints

You’re here for one reason: you want to make your prints look like they weren’t printed in the first place. This guide is aimed to newcomers looking to get their prints smooth and primed for painting. In addition to making prints look better, post-production can help fix minor imperfections, saving time to reprint it. This guide will go over a range of tools and techniques for finishing your 3D prints and then dive into how you can implement them for your next project. Sometimes all you need is filler primer and sandpaper to finish a print. Other times, you’ll need more.

Don’t hesitate to reach out with questions or share your own techniques by emailing me at FjoriMakes@gmail.com

Chapters

i. Tools

  • X-Acto Knife
  • Palette Knife
  • Needle Nose Pliers
  • Sculpting Tools
  • Sandpaper
  • Dremel
  • Filler Primer
  • XTC 3D/Resin
  • Spot Putty
  • Free Form Air
  • Bondo
  • Adhesive

 

ii. Cleaning Up Prints

  • Cutting Off Blemishes, Zits, and Blobs
  • Removing Brims
  • Taking Off Supports

 

iii. Processes

  • Adhesive
  • Free Form Air / Bondo
  • Resin/Epoxy
  • Sanding
  • Filler Primer

 

 

Before I dive in, I’d like to add that these tools and processes are just what happen to work for me through trial and error. My goal is to familiarize you with what’s available and their results, so that you can make the best decision for your project. There are more tools and methods available than just the ones I mention below.

i. Tools and Materials

In this section I’ll be going over all of the tools and materials I use. 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.

  • X-Acto Knife

Whether there’s a brim that needs to be removed, or removals of a few blobs and zits, a little trimming with X-acto knives can eliminate the need to reprint.

  • Palette Knife

Palette knives are quite useful for all stages of 3D printing: they can help remove prints off the bed, allow you to apply Bondo more accurately, and still serve their original purpose: mixing paint.

I recommend getting them in a pack so you can assign a specific role for each one. I use #2 specifically for removing prints, and #5 for Bondo application.

  • Needle Nose Pliers

These are useful for taking off large chunks of supports, as well as cutting filament.

  • Sculpting Tools

These tools are useful for removing Bondo or Free Form Air out of small nooks and crannies. 

  • 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 when it comes to sandpaper. If there’s a need for it, it exists. Below are a few examples I use:

– 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. Once the surface has worn out, I use it as a base for sheets or rolls of sandpaper which I’ll get into next.

– 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.  I go through 220 grit sandpaper the most.

– Sanding Sticks

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

 

  • Dremel

Dremel is a handheld motorized rotary sander. Even the lowest setting is usually too much power to work on 3D printed parts without damaging them. There are exceptions and they can come in handy for the right project. I generally don’t bust out my Dremel unless it’s a large scale project and sanding by hand won’t do. It also gets some use when working with ABS.

  • Filler Primer

This is truly a godsend for finishing 3D printed parts – it does a fantastic job of filling in between layer lines. Regardless of whether or not you use resin, filler primer is a necessity for post-production: it helps your paint job stick to the print.

You can buy several different types and colors at your local Home Depot.

 

  • XTC 3D / Resin

XTC 3D is a resin that nestles in between layer lines. It strengthens a print and adds structural integrity.  This is my preferred method to fill in striations and generally helps remove 50% of sanding work versus only using FFA or nothing at all.

(Note that this product is currently only available in the US.)

 

  • 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 cracks.

 

  • Free Form Air

Free Form Air (FFA) is an epoxy dough that’s a great alternative to Bondo. It has a soft, play dough-like texture. This item is somewhat pricey, 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 tends to require 6-10 hours to dry, whereas Bondo only needs about 20 minutes to an hour. FFA is also great for small spaces and filling in between layer lines. It tends to crumble when attempting to spread over a large area. Bondo is generally a better option for things that have large, broad areas that need to be covered (like a sword).

 

  • 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 to using it. 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 take adequate safety precautions like wearing disposable gloves.

 

  • Adhesive

The majority of large prints, I use epoxy glue. It has a powerful bond with a working time of 5 minutes. The only time I do not use it is if the print is quite small. In that case, superglue will do.

For intricate prints or attaching pegged parts, superglue will do the trick. It comes with small nozzles ideal for reaching into small miniatures and detailed areas.

 

 

 

ii. Cleaning Up Prints

Now that you are aware of some of the tools available, let’s get started with post-processing.

  • Taking Off Brims

Using the needle nose pliers, hold on to one end and slowly turn the pliers away from you, curling it. The rest of the brim can usually be peeled by hand. 

With the X-acto knife, clean up any leftover remnants.

 

  • Cutting Off Blobs and Zits

Blobs and zits can usually be fixed with small tweaks to your print settings. For troubleshooting prints, I recommend S3D’s guide here: https://www.simplify3d.com/support/print-quality-troubleshooting/

If you don’t want to reprint, use your X-acto knife. Place your thumb on the grip portion of the knife and gently guide the blade against the blob.

  • Removing Supports

If you printed with supports and your print has a lot of small details, I advise using hobby scissors or even just your hands to remove them. Otherwise, the needle-nose pliers work well.

Grab on to a large section of support and twist. Depending on the software that generated the supports, the supports may come off in one go. Keep going until all excess material is removed.

Again, X-acto knife also comes in handy here when removing blemishes from poor support surfacing.

 

iii. Processes

This chapter is intentionally labeled ‘Processes’ rather than ‘Step by Step’ because you will not always need to go through each step for each project. Every print is different and will require different finishing techniques.

The print should be clean and ready to go – in this section I’ll explain the different processes for post-production. You may find yourself going back and forth between a few steps.

 

  • Assembling Parts With Adhesive

If you’re printing a particularly large project, sometimes splitting a print into parts is the only option. Gluing parts is not as arduous as it may seem. If you have the option to work with the source 3D model, adding pegs leaves less room for error. TinkerCAD.com offers a quick way to do so (I’ll eventually make a separate guide for this).

Begin by sanding the two ends that will be glued together with a low grit sandpaper. The more surface contact between the two parts and makes the glue bond the better. This process is akin to scoring clay to attach pieces together. 

Once it resembles the photo below, the print is ready for adhesive. 

 

Get your glue of choice ready. If using superglue, you’ll only need a small amount for it to be effective.

 

Once the glue of your choice has been applied to one surface, hold the pieces together tightly. Use clamps to ensure a tight seal if needed.

 

Applying Gorilla Glue

If you printed a piece in multiple parts, sometimes there may not be enough surface area for superglue to be effective. Gorilla glue comes in handy to brute force pieces together. I tend to overuse it to be on the safe side and then use an X-acto knife to cut off any excess that may have oozed out after it has dried.

Filling In Gaps and Blemishes

Your parts are all glued together, yet there are still gaps between print. Sometimes prints warp and this can cause significant gaps between pieces. There are several choices aside from reprinting with a brim or raft: You can apply spot putty, Free Form Air, or Bondo depending on your needs.

 

Spot Putty

Spot/Glaze putty is a soft, clay-like putty. Not only does it dry quickly, it also doesn’t require any mix (unlike FFA or Bondo). I use this exclusively to fill in gaps between assembled pieces, but its texture leases itself to many uses.

Here are my main uses for spot putty:

  • Covering blemished support areas
  • Hiding seams between assembled pieces
  • Filling gaps in a hurry

To start working you’ll need:

  • Disposable Gloves
  • Breathing Protection
  • Palette Knife (optional)

Either dab a small amount on your finger, or brush the tube against the area you want to fill. It’s alright if it looks rough – nothing a little sanding can’t fix!

 

I personally use my fingers to fill in the desired areas. It’s wet sandable, but my preference is to wait until it’s dry.

For most projects, you can start using filler primer at this stage and begin sanding. Regardless of what filler I pick, I still finish 9/10 prints with XTC 3D.

 

Free Form Air

Free Form Air is a soft, doughy epoxy. It is similar to Bondo with its effects, but there are situations that it can outpreform its competitors.

 

So when do you want to use Free Form Air (FFA)? Here are some samples of when I utilize this soft epoxy over alternatives:

  • Between pieces that I glued together
  • Striations on organic shapes
  • Warping during prints

 

To start working you’ll need:

  • Free Form Air Part A and Part B
  • Disposable Gloves
  • Palette knife (optional)

 

Take equal parts Part A with Part B.

Mix thoroughly until consistent in color and feel.

Use your finger to guide the putty in the large gaps. Allow some excess so that it can be smoothed perfectly later with sandpaper.

You can also rub the epoxy between layer lines. Think of it as applying a thick lotion.

Sometimes Free Foam Air can get into spaces you don’t necessarily want it. Use your sculpting tools to take out any unwanted mixture.

FFA is also good if you have poor support surface. This is an easy fix: take a small amount of FFA with your index finger.

Start at the beginning of the rough area and take the mixture. Smear it with your finger all the way through the until the entire rough area has been covered. You may also use a palette knife: just make sure to go slowly as FFA tends to crumble easily.

It may look rough after application. This is fine as it will be sanded after drying. Allow up to 8 hours to fully dry before sanding (it depends on how thick you applied it).

 

 

 

Bondo

Bondo is similar to FFA in that it’s a soft putty-like filler. The texture is less viscious compared to FFA. While it’s cheaper and takes less time to dry than FFA, it comes with some possible drawbacks. It produces much more fumes and tends to be harder to manipulate into small spaces.

Here are other uses I get out of Bondo:

  • Covering large, flat areas with the use of a palette knife
  • Filling gaps in a hurry

 

To start working you’ll need:

  • Bondo and Cream Hardener
  • Disposable Gloves
  • Palette knife (optional)

Just look at this print – what a disaster! No need to waste the 5 hours of printing though.

On a piece of cardboard or recycled material, scoop a small amount of Bondo with your palette knife. Then dab a very small amount of cream hardener. Mix thoroughly with the palette knife until the cream hardener has been fully incorporated. There are colored versions of the hardener available to make mixing more visible.

Take a small amount of the mixed Bondo with the bottom tip of the palette knife and distribute a small amount onto the area you’re trying to patch.

Continue doing so until everything you need is covered. As long as you’re wearing gloves (which you should be!) you can also use your finger to help shape some of the Bondo.

After you finished applying Bondo, it will still look ugly (and that’s okay!) Allow up to an hour to dry in a well ventilated area. After it has properly dried, you can begin sanding. 

Start with an 80-100 grit sandpaper to remove the excess chunks. Then go with 220 begin sanding the remaining dried Bondo. The picture below is what it looks like after a cycle or two.

 

 

 

XTC 3D

XTC 3D is a resin that fills in the majority of the striations in your print. Even a small amount can be incredibly effective at elimating much of your sanding time.

To start working you’ll need:

  • XTC 3D Part A and Part B
  • Disposable gloves
  • Breathing protection
  • Foam brush
  • Stirring utensil
  • Metallic container / Tin foil

To begin working with XTC 3D, make sure you have a small plastic measuring cup, a foam brush, and a mixing stick. Make sure you have an exit strategy – this resin is quite messy, so you need to figure out where to cure your print. I tend to rest mine against a few measuring cups once I’m done applying XTC 3D. 

First step is to put on disposable gloves. Resin is nasty to get on clothes and your workdesk, even worse if on your skin. Please take proper safety precautions and read the safety warnings on the labels.

To begin, shake both bottles.

Mix these bottles separately. First, take 2 Parts A and measure out how much you want. Err on the side of using less – a little goes a long way with XTC and you can always mix more!

Scoop that into your metal tin or tin foil bowl.

Mix 1 Part B

Do the same step as you did with Part A and empty it into the container.

Use your mixing stick (or an old paintbrush if you’re a cheapo like me) to stir the two solutions together. The end consistency should resemble molasses. PS You don’t need this much for one print! I coated several large prints with this batch.

Dip the tip of your foam brush into the mixture. You don’t need to gob it on.

Lightly brush the mixture onto the print.

Coat your entire print with the mixture. 

Give at least 8 hours for it to set before beginning sanding. Setting the print in direct light will slightly speed up the process.

 

 

Sanding

Prepare your arms: As mentioned earlier, sanding is usually the most time consuming part of post-production. Using resin or epoxy can help reduce in how much sanding is required, but in the end it’ll probably be your forearm’s endurance making the call.

Get a sanding block or strip of low grit sandpaper (80-150 is fine) and start sanding everywhere until the print has roughed up a bit. This grit is usually labelled “removal” for stripping paint and the like. Think of it as removing the space between the lines. The goal is to get the lines starting to blend into each other. This also helps get rid of excess Bondo or similar gunk.

Now get 220 grit sandpaper and begin with the same process. At this point, the print should be smoothing out significantely.

Once everything has an even surface to the touch, it’s ready to be sprayed with the first coat of Filler Primer. 

 

Filler Primer

Regardless of whether or not you used resin, filler primer is an essential tool for 3D printing. Not only does it fill in cracks, but it’s necessary for your paint job to adhere to your print. As a note, while it’s absolutely feasible to spray your print directly without applying any sort of filler, there will probably just be more sanding required up front. The process rarely ends after one application of primer – you will be going back and forth between this step and the previous. Patience pays off here.

Before spraying, here’s what you’ll need:

  • An outdoor area, ideally enclosed by a cardboard box or something similar.
  • Breathing protection
  • Something steady to hang or hold the print while spraying

Do not spray any sort of paint indoors!

Prep your outdoor work area. Shake the can for 30 seconds or so. Aim the primer about 10 inches and away from your face and spray the print on all sides until it has been thoroughly coated. I personally use the Flat Red primer.

Allow the print to dry.

Here is an example of the first coat of primer applied. Now that the primer has dried, you guessed it, time to sand again. This process is generally repeated 3-5 times.

After first cycle of sanding with 80-200 grit sandpaper. It’s okay if you sand the primer off completely in some areas. The print will be sprayed again.

 

After the primer has dried, I start with 220 grit sandpaper until the layer lines have completely disappeared. Around the third time, I work up in intervals of 500 until the print feels completely smooth. Larger prints can take up to 5 or more cycles.

After using 1000 grit sandpaper

Here is my personal process:

Sand > Spray Filler Primer > Sand with 220 > Spray Filler Primer > Sand with 220 again until completely smooth > Spray Filler Primer > Sand with 200 to 400~ > Spray Filler Primer > Sand with 800 grit and work up to 2000.

If there are no more visible layer lines, continue using higher grit paper in, increasing your grit by -500 intervals. Most prints can be finished with 2000 grit.  

If your print’s form is smooth and even, it’s time to start painting!

 

I’d love to see what you come up with – share your creations by tagging @Fjorimakes on social media.

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