3D Print Finishing Explained – How to Fill, Paint & More
At 3DSourced we’ve covered everything 3D printing and 3D since 2017. Our team has interviewed the most innovative 3D printing experts, tested and reviewed more than 20 of the most popular 3D printers and 3D scanners to give our honest recommendations, and written more than 500 3D printing guides over the last 5 years.
Whether you’re delving into 3D printing as a business venture or striving for a smooth surface on your hobbyist prints, this guide will walk you through the optimal 3D print finishing techniques to achieve a superior surface quality.
I’ll share my proven methods for post-processing, from sanding to vapor smoothing, to help you transform your printed parts into polished masterpieces.
- How to Finish 3D Prints: Basic Principles
- How to Remove Supports From 3D Prints
- Filling Gaps in 3D Prints
- How to Sand 3D Prints (& Polishing)
- Vapor Polishing
- How to Paint 3D Prints (The Right Way)
How to Finish 3D Prints: Basic Principles
One of the best ways to minimize the amount of post-processing work cleaning up 3D prints is by making smart choices from the start of the printing process.
Here are the slicer settings and materials I recommend to set yourself up for smooth, shiny surface on your 3D printed objects.
Think of layer height as the resolution of your 3D prints, similar to pixels in a photograph or a video.
Choosing a thicker layer height translates into broader layer lines and stepping layers, producing less fine detail and a more visible layer structure. On the other hand, a thinner layer height allows for a greater level of detail, seamlessly blending layers into one another and reducing layer thickness for a more uniform look.
If your end goal involves painting your 3D printed parts, opting for a lower first layer height is advisable. While it may increase your printing time, it saves significant effort in sanding down layer lines before applying your coats of paint.
Print speed, the rate at which the print head extrudes filament, dictates both your print’s quality and the time you spend 3D printing.
Simple designs can bear higher speeds with no significant quality loss, while detailed, complex models require slower print speeds for precision. It pays to experiment with your print speed to see what works best for the job you’re printing.
Infill and Shell Thickness
The shell is your object’s outer wall. A higher shell thickness setting results in thicker object walls, ensuring a more robust print.
You’re going to be sanding 3D prints after printing in order to obtain the smoothest surface possible. This means that your object is going to need to be a bit more robust in terms of thickness and infill to accommodate for the extra material to be removed during the sanding process.
Fill density, or infill, defines the amount of material inside the object’s outer shell, indicated as a percentage. A 100% fill density yields a solid print with no internal voids, while 0% leaves the interior empty.
While adjusting fill density can conserve filament and expedite print times, it’s a balance that also affects the print’s strength and weight. A higher fill density results in a sturdier, heavier print. If you finishing process includes sanding, a higher infill will ensure your 3D printed object withstands the pressure without damage or deformation.
For deeper insight into slicer settings, explore our dedicated article here.
Choosing the Right Filament
Beyond slicer settings, your filament’s quality significantly influences your 3D print’s finish. Lower quality filament often contains impurities, affecting its melting, extrusion, and cooling properties, which ultimately influence the end result’s quality.
Cheaper filament may have an irregular diameter that can lead to under-extrusion and over-extrusion during printing, causing serious surface flaws that need extra finishing effort.
Consider using a color of filament that will closely match the desired color of the finished object to reduce the amount of painting required post-printing. This is less important for those truly professional-grade finishes though, because of the amount of primer paint coats needed.
How to Remove Supports From 3D Prints
3D printers operate from the bottom up, which poses challenges for designs with overhangs – upper areas in the model lacking underlying material. These overhangs require support structures for printing, and these supports must be removed before the finishing process can begin
Non-soluble support structures can be removed with flush cutters or an Exacto knife. Following removal, lightly sand with 200 or 400 grit sandpaper to smooth any remaining marks left from the support structures.
You’ll notice in the image above, the model was printed upside down so that minimal support structures would be left – meaning less work to remove them. It’s easier to smooth PLA on large, simple areas like the top of the helmet than uneven sections underneath
Soluble support structures, typically made of PVA or HIPS, can be dissolved. Submerging the object in the appropriate solvent – water for PVA or Limonene for HIPS – will dissolve the support structure. PVA usually dissolves within a few hours in water, while HIPS takes around 24 hours in Limonene.
Filling Gaps in 3D Prints
Prior to sanding and painting 3D prints, consider applying a 3D print filler to small gaps, cracks, or underfilled areas. Typically, these fillers are variations of easy-to-handle epoxy resins that can be sanded post-application.
Although it won’t yield the high-quality finish we’re targeting in this guide, it’s a convenient solution for a quick-fix enhancement.
The neater you are filling the gaps, the less sanding you’ll need to do afterward – so try to be as neat as possible. You’ll also need to leave it to cure for at least a few hours after filling.
How to Sand 3D Prints (& Polishing)
Sanding is a simple and easy way to finish processing a 3D print, depending on the material in question. Some printing materials respond well to sanding, with other materials it may not be an option.
For example, PLA and ABS can can achieve a beautifully polished finish through sanding alone. With harder materials, like Polycarbonate and PMMA, it’s better to go for acetone vapour polishing (which we will discuss in more detail later.)
Quality filament is crucial— substandard filaments can make achieving a smooth surface more challenging.
The sanding process demands patience, especially with sanding PLA prints that can soften and become gummy under friction due to their low melting temperature.
Pro tip: Consider using wet sanding with waterproof abrasive sheets like “Wet & Dry”. This method helps mitigate frictional heat, enabling effective material removal without distortion.
Use a circular motion and moderate pressure with the coarsest grit sandpaper, aiming for a uniform surface. Gradually increase the grit size (from 200 to 400, then 600, and so forth), patiently repeating the sanding process. The slow removal of unwanted rough surface features and preservation of desirable elements are key to achieving your desired surface finish.
Start with coarser sandpaper such as 200 grit, and progressively work your way to finer grades, up to 3000 grit. As you ascend the grit scale, spend more time on lower grit sizes to get that mirror smooth-finish.
Vapor polishing utilizes solvent vapors to smooth the surface of 3D printed parts. It involves placing both the print and a suitable solvent in a container, with the object suspended above the solvent. The container is then covered (not airtightly) and heated, causing the solvent to vaporize.
This process resembles water heating to produce steam. The heated solvent transitions from a liquid to a gaseous state, surrounding and interacting with the print’s surface. The solvent vapor dissolves the surface uniformly, eliminating unwanted features.
Upon achieving the desired finish, the print is removed and air-dried, revealing a professionally finished object.
Much like sanding, boosting shell thickness and fill density before printing is important to enhance the object’s robustness, making it more resilient to vapor polishing. The process softens the print’s surface layers, causing them to merge and create the desired finish. If insufficient top layers exist, vapor polishing can expose the underlying surface and compromise the results.
While commercial-grade equipment like cloud chambers are typically used for vapor polishing, hobbyists can replicate the process with household items. A wide-mouthed glass container or a saucepan can hold the solvent, while an aluminum foil platform suspends the print. Controlled heating can be achieved using your printer’s heated bed, too.
Caution: Different printing materials require specific solvents, some of which can be highly toxic. For instance, PVA is water-soluble, HIPS dissolves in Limonene, ABS and PMMA in acetone, PLA in tetrahydrofuran (THF), and Polycarbonate in dichloromethane.
Vaporizing these solvents presents hazards— they can be easily absorbed by the body through inhalation or contact, and many are flammable. Therefore, vapor polishing should be performed in a well-ventilated environment, wearing a respirator, protective clothing, and gloves.
How to Paint 3D Prints (The Right Way)
Whether you’re working towards an impressive prototype for a Kickstarter campaign, or striving to create an awe-inspiring 3D printed piece, painting will add the finishing touch.
Achieving shiny and smooth paintwork requires two steps – priming and then painting. Before we delve into either, let’s look at the steps you need to take to guarantee stellar results once you start painting.
I always secure my 3D printed object to a painting block with a dowel rod during the painting process for seamless finishing.
To start, drill a small hole in a less visible area of your object that aligns with the dowel’s diameter. Securely insert the dowel in this hole, or use an existing indentation or opening on your object to hold the dowel.
Next, create a support system for the dowel by drilling a hole in a wooden block using the same drill bit size. Insert the other end of the dowel into the wood block. Now you’re set up for evenly applying smooth coats of primer and paint without leaving any smears or fingerprints.
You could also consider setting up a makeshift spray booth by draping plastic sheeting around three sides of the working area. This helps contain the paint spray and significantly reduces dust and debris sticking to your object while the paint or primer dries.
Consider wearing an approved respirator and nitrile gloves since spray paints often contain solvents that could be harmful if inhaled.
How to Prime 3D Prints Before Painting
Priming uses a special type of paint that comes in neutral colors, designed to leave uniform surface for paint to bond to. It comes in both brush-on and spray-on, but I recommend spray-on. It promotes a smooth coat that doesn’t leave conspicuous brush marks on your 3D prints/
For best results, use a primer and paint combo that is plastic-compatible and ideally from the same brand. I recommend high-build filler primers from brands like Krylon, Tamiya or Montana.
Before starting the priming process, swirl the primer can for two to three minutes to helps dissolve the pigment into the solvent. Don’t shake the can as it will make the pigment mix instead of dissolve, causing surface bubbles during spraying. Once the metal ball within the can moves smoothly, you’re all set to prime your object.
Begin priming from a distance of about 15 to 20 cm (6 to 8 in.) from the object’s surface. Use quick, short strokes, starting before the object begins and ending past its end while rotating as necessary.Aim for a thin initial coat to be build upon rather than over-spraying.
Once that has dried, it’s time for the second coat. Again, use of short, swift strokes. In most scenarios, two coats of primer is enough to get an even surface that optimizes paint adherence.
After the second coat drys, the next step is to buff and polish the primer coat. I often use nail buffing sticks, which you can find in any chemist or drug store. Use a circular motion with uniform pressure. Post-buffing, wipe the object gently with a tack cloth to rid it of any residual dust.
Now that the primer coat is buffed and polished, it’s time for the painting process. There are three steps – undercoating, top coating, and clear coating.
An undercoat should not be confused with the primer coating. The two serve entirely different purposes.
The primer serves as a preparatory coat, ensuring even paint adhesion and preventing blotching. On the other hand, the undercoat serves to conceal the neutral primer shade.
Typically black or white, black undercoats lending themselves to darker colors, and white undercoats complementing lighter shades. The chosen undercoat subtly shines through subsequent coats, enriching the final finish.
As with the primer, ensuring the paint is well-mixed is key. Spend a few minutes swirling the can until the pigment merges with the solvent, waiting for the metal ball to move smoothly within the can.
To apply your initial undercoat, maintain a distance of 15 to 20 cm between the nozzle and your 3D printed part. Using swift, short strokes, layer the paint on your piece, starting the stroke before the object and ending past it. As you evenly apply this thin layer, rotate the object on the painting block to ensure an even coating.
Following the drying of the first layer, buff and polish the surface as you did after the primer application. Afterward, you’re set to apply the second coat in the same manner as the first. Once dry, a final round of buffing and polishing readies your piece for the next stage.
Now it’s time to apply the top coat. Top coating isn’t about complete coverage – it is the final decorative flourish
Ideally, the top coat will be a color that contrasts with the color of your undercoat. The top coat will add gradients of color which will allow the contrast of the undercoat to show through.
The steps for applying the top coat mirror those of the primer and undercoat. Keep the same 15 to 20 cm distance between the paint can nozzle and your piece, and use quick, short strokes while rotating your object on the paint block for even coverage.
Polishing and buffing between the first and second top coats ensures a smooth finish.
Less is more with the top coat. The aim is to allow the undercoat’s contrast to shine through, enhancing the sense of depth. Consequently, take care to apply smooth, thin layers.
Upon the drying of the first top coat, evaluate its contrast with the undercoat, and then apply the second coat accordingly. This multi-layer approach creates a beautiful contrast and depth, making your object stand out with its smooth and professional-looking coating.
Clear coating, or finish coating, marks the grand finale of the painting process. It serves a dual purpose: adding a final touch that enhances the object’s aesthetic while also safeguarding your finishing work under a protective layer.
The application process for the clear coat is the same as the previous coatings. Mix the coating well before applying using quick, short strokes.
The smooth coats can be applied once or twice; a second glossy coat can enhance the gleam.