Using PLA with an FDM 3D printer can lead to excellent results. The process is fast and affordable, and PLA filament can produce functional printed parts with a good level of strength and stiffness.

But FDM has its drawbacks too. For instance, it produces visible layer lines, which can compromise the appearance and function of printed parts. Furthermore, PLA can be more difficult to work with than other 3D printing materials like ABS, as it can’t be smoothed with acetone.

Fortunately, there are several ways to smooth PLA 3D prints, ranging from beginner-friendly techniques like sanding to advanced methods like solvent application. This article looks at the best ways to smooth PLA prints, showing you which technique will work best in a given situation.

Untreated FDM parts have visible layer lines

How to Smooth 3D Prints

Although FDM parts can’t easily attain the smoothness of, for instance, SLA parts, there are a number of post-processing techniques that can transform bumpy prints into smooth, rounded, and glossy parts. Some of these options involve adding material to the print (primer spray, epoxy resin), while others involve removing material (sanding, chemical smoothing).

Of course, adding a smooth surface to PLA printed parts will add time and cost to a project, but the results are often worth it. Not only does smoothing improve the appearance of the prints, it allows for less strict printing parameters — greater layer height, for instance — which in turn leads to faster printing.

Below are some of the most common methods for smoothing PLA prints. They should be carried out after preliminary post-processing steps, i.e. the manual removal of supports and large sections of excess material.

Sanding

The most common method for smoothing PLA prints is sanding. This method is highly practical, as it is inexpensive and poses no health risks. However, it is also one of the most time-consuming ways to smooth a print and is not especially suitable for fine features or reaching into deep crevices.

Sanding can be approached in different ways depending on the purpose of the parts and the desired level of smoothness. Large parts may require a sanding block (a piece of wood with sandpaper wrapped around it), while small parts and fine features may be suited to nail files. Though highly efficient, electric sanders are not recommended for PLA parts, as the heat they generate can melt the PLA and warp the part.

Sanding PLA parts requires sandpaper in different grit sizes

For most prints, sanding begins with coarse 200 or 400 grit sandpaper — either loose or wrapped around a sanding block — worked in circular motions over the surface of the part to remove the most extreme protrusions. After the whole part has been sanded, a slightly finer sandpaper is used, working up in increments to 3,000 grit.

Although it may seem like a big undertaking, it is worth obtaining sandpaper in the following grit sizes and sanding for a few minutes with each size:

• 200

• 400

• 600

• 800

• 1,000

• 1,500

• 2,000

• 2,500

• 3,000

With the finest grit sandpaper, a small amount of water can be applied to produce an even smoother finish.

Polishing

For high-shine PLA parts, the sanding stage (which gives a smooth but matte finish) can be followed by a polishing stage, using a cotton cloth and liquid plastic polish. This method is not suitable for smoothing highly uneven surfaces on its own; it is more of a finishing technique.

The technique for polishing 3D printed parts is similar to sanding: with polish applied, the wet cloth should be moved in even circular motions over the surface of the part until it is smooth and shiny.

Priming

Sanding alone can reduce the appearance of layer lines by shaving off the raised “steps,” but another approach is to fill in the crevices with primer spray then sand down the excess primer afterwards. This method works well because primer is easier to sand than PLA itself.

Priming is the most time-consuming way to smooth PLA parts, because primer must be applied in between each round of sanding then left to dry. However, it produces better results than sanding alone.

When priming a PLA print, the part is placed in a well-ventilated area and sprayed evenly with a very thin coat of the primer. One or two more coatings can be added before the first round of sanding. Alternate priming and sanding is then carried out, finishing with 3,000 grit sandpaper.

Priming and sanding is usually followed by spray painting, as uncoated primer will be susceptible to impact and environmental damage. Also note that very deep cavities should be filled with plastic filler rather than primer.

Epoxy Resin

A different type of coating for smoothing PLA parts is epoxy resin, which comes in two parts (resin and hardener) that must be mixed together before use. Unlike primer, which comes in a spray can, epoxy resin can be applied to parts using a paintbrush and is suitable for filling even the deepest holes.

To ensure good adhesion of the epoxy, it is recommended to first carry out wet sanding on the PLA parts with fine-grit sandpaper. Once the part is dry, the epoxy can be applied in generous amounts and spread evenly across the part surface. Further layers of epoxy can be added until the desired level of smoothness is reached.

Note that the epoxy resin coating may be weaker than the PLA underneath, and bits of the coating may flake or chip off over time.

Generic epoxy resins can be used, but there are also mixtures designed specifically for smoothing 3D prints, such as Smooth-On XTC-3D. 

Epoxy resins like Smooth-On XTC-3D can improve PLA smoothness

Solvents

One of the big disadvantages of PLA is its resistance to acetone smoothing, a relatively safe-to-use chemical smoothing process suitable for materials like ABS.[1]

PLA prints can only be chemically smoothed using stronger chemicals that require laboratory equipment like nitrile gloves, protective eyewear, fume hoods, and a well-ventilated environment. These chemicals dissolve the top layer of material, resulting in a smooth finish — but they can dissolve other things too, such as plastic containers, work surfaces, and human skin.

Experienced users may smooth PLA parts with chemicals like tetrahydrofuran, dichloromethane, or chloroform.[2] However, these substances are difficult to obtain and can be dangerous to use.

A safer option is ethyl acetate, a solvent found in ordinary nail polish remover. Nail polish remover can be applied to PLA parts using a cloth or cotton swab, though it is limited in its effectiveness. In general, because of the risks associated with the appropriate chemicals, PLA is not highly suited to chemical smoothing.

Vapor Smoothing

A safe and effective method of chemical smoothing PLA parts is to use an automated vapor smoothing machine such as the Polymaker Polysher, which creates a mist of alcohol to gently remove the top layer of a printed part.[3] Unfortunately, the Polysher is only compatible with Polymaker’s modified PLA products, PolySmooth and PolyCast, which dissolve in alcohol due to the presence of additives.

Researchers have found that ethyl acetate can also be used for the effective chemical vapor smoothing of PLA. According to one study, use of ethyl acetate vapors can "achieve almost a 90% of roughness reduction, with several advantages in terms of toxicity, quantity of solvent, and treatment time."[4]

Heat Gun Smoothing

Though not the most precise method of smoothing printed objects, using a heat gun to melt the outer surface of a part is a fast and easy way to remove layer lines. A heat gun is a device that resembles a hairdryer but is capable of reaching much higher temperatures. (A hairdryer can also be used with some effectiveness.)

When using a heat gun to smooth PLA parts, it is important to move the gun evenly to ensure consistent heating across the part surface (or place the part on a revolving platter and keep the heat gun steady).

This technique only works on medium-size or large parts that do not have fine features or hollow sections.

Tips for Successful PLA Smoothing

Achieving optimal results when smoothing PLA prints requires patience, attention to detail, and adherence to best practices. The following tips can help ensure a successful smoothing process and improve the overall quality of your 3D printed objects.

1. Start with a high-quality print: The quality of the initial print plays a significant role in the success of the smoothing process. Ensure that your 3D printer is properly calibrated and maintained, and use high-quality PLA filament to minimize print defects and imperfections.
2. Choose the appropriate smoothing technique: Selecting the right smoothing technique for your specific project is crucial. Consider factors such as print size, complexity, material compatibility, and desired surface finish when choosing a method.
3. Follow safety precautions: Some smoothing techniques, such as chemical smoothing and vapor smoothing, involve the use of potentially hazardous chemicals or solvents. Always work in a well-ventilated area, wear appropriate personal protective equipment, and follow the manufacturer's safety guidelines when using these methods.
4. Be patient and methodical: Smoothing PLA prints can be a time-consuming process, especially when using techniques like sanding or applying epoxy coatings. Take your time and work methodically, focusing on one area at a time and ensuring that each step is completed thoroughly before moving on to the next.
5. Inspect and evaluate your progress: Regularly inspect your print during the smoothing process to assess your progress and identify any areas that may require additional attention. This can help prevent over-smoothing or uneven results.
6. Allow sufficient curing or drying time: When using techniques that involve solvents, epoxy coatings, or heat treatment, it is essential to allow sufficient time for the print to cure or dry before handling or using it. Rushing this step can result in a compromised surface finish or damage to the print.

Post-Smoothing Finishes

After smoothing  PLA prints, additional finishing techniques can be applied to further enhance their appearance and durability. These techniques can add a professional touch to your prints and ensure they meet the desired standards for your project.

Painting

Painting is a popular finishing technique that can be used to add color and texture to smoothed PLA prints. Acrylic paints are commonly used for painting 3D printed objects due to their compatibility with PLA and ease of application. Application of a primer before painting will ensure proper adhesion of the paint, and multiple thin layers of paint should be applied, allowing each layer to dry before applying the next.

Polishing

Polishing is a technique used to further refine the surface finish of smoothed PLA prints, particularly after sanding or chemical smoothing. Polishing compounds, such as automotive polishing compounds or plastic polishes, can be applied using a soft cloth or buffing wheel to achieve a high-gloss finish. Work gently and methodically, avoiding excessive pressure or heat buildup that could damage the print.

Clear Coating

Applying a clear coat to your smoothed PLA prints can provide additional protection and enhance their appearance. Clear coats, such as spray-on lacquers or varnishes, can be applied to the print's surface to create a protective barrier against scratches, UV damage, and environmental hazards. When applying a clear coat, follow the manufacturer's instructions and allow sufficient drying time between coats.

Conclusion

As we have seen, there are several ways to smooth PLA prints and reduce the appearance of layer lines. Choosing the right smoothing method (or methods) depends on the nature of the 3D printing project and your priorities. In general, we would recommend:

• Priming and sanding for most PLA parts - effective, safe, cheap

• Epoxy resin or polishing for cosmetic parts - high-shine finish

• Vapor or heat gun smoothing for rapid parts - fast, least labor-intensive

For parts suited to vapor smoothing (fragile, intricate, etc.) it may be helpful to try printing them in ABS instead of PLA. Alternatively, resin 3D printing technologies like SLA and DLP, though more expensive than FDM, are much better at producing smooth parts without layer lines and do not require much post-processing at all.

Frequently Asked Questions (FAQs)

Can I use acetone to smooth PLA prints?

Acetone is not recommended for smoothing PLA prints, as it does not react with PLA material. Instead, use alternative solvents like ethyl acetate for chemical smoothing of PLA prints.

How long does it take to smooth a PLA print using sanding?

The time required for sanding a PLA print depends on the size and complexity of the object, as well as the desired level of smoothness. It can range from a few minutes for small, simple prints to several hours for larger, more intricate objects.

Can I use a heat gun for heat treatment of PLA prints?

While a heat gun can be used for heat treatment of PLA prints, it is important to maintain precise temperature control and avoid overheating the print, which can cause warping or melting. Do not focus the heat in any single area for too long.

Can I paint my PLA print directly after smoothing?

Before painting a smoothed PLA print, clean the part surface and apply a primer to ensure proper adhesion of the paint. Allow the primer to dry according to the manufacturer's instructions before applying the paint.

References

[1] Tuazon BJ, Espino MT, Dizon JR. Investigation on the effects of acetone vapor-polishing to fracture behavior of ABS printed materials at different operating temperature. In Materials Science Forum 2020 (Vol. 1005, pp. 141-149). Trans Tech Publications Ltd.

[2] Zhang B. Does Acetone Dissolve PLA? [Internet]. EcoReprap. 2021 [cited 2022Mar1]. Available from: https://ecoreprap.com/dissolve-pla/

[3] Kočí J. Improve your 3D prints with chemical smoothing [Internet]. PrusaPrinters Blog. 2020 [cited 2022Mar1]. Available from: https://blog.prusaprinters.org/improve-your-3d-prints-with-chemical-smoothing_36268/

[4] Lavecchia F, Guerra MG, Galantucci LM. Chemical vapor treatment to improve surface finish of 3D printed polylactic acid (PLA) parts realized by fused filament fabrication. Progress in Additive Manufacturing. 2022 Feb:1-1.