How to Smooth PLA 3D Prints: A Complete Guide

Why Should You Smooth PLA 3D Prints?

You may be wondering why you would want to smooth down 3D prints, to begin with. There are actually quite a few benefits, such as:

• Finishing the surface of a print makes it appear that much more like a professional, store-bought product.
• Furthermore, finishing the piece can help increase durability, preventing damage, and helping with longevity.
• Smoothing can also fix or cover up any slight imperfections that occurred during the printing process.

Methods for Smoothing PLA 3D Prints

While ABS is easy to finish and can accommodate most methods, PLA prints are a little fussier when it comes to materials. One of the simplest and most common ways of finishing 3D prints is acetone smoothing, but since PLA is not reactive to acetone, the technique is just not feasible when printing with PLA.

Sometimes impure PLA can be smoothed with acetone, but that is only because it is a cheaper material blended with other plastics that react to acetone. Higher quality PLA will not be affected by acetone.

Luckily, there are plenty of other methods to take advantage of if you are looking to smooth out your print’s surface, including:

• Sanding
• Heat gun
• Polishing compounds
• Liquid metal polish
• Painting
• Chemical solvents

You might find that your preferred smoothing method includes a combination of the methods listed above. Feel free to experiment on test pieces to figure out what works best for you.

However, in general, to smooth PLA filament, you may need to add or subtract different materials until it achieves your desired smoothness.

Alec Richter probably said it best: “…finishing consists of; a fine balance between adding and subtracting material to achieve the desired surface finish. The key is to remove enough material to hide some striations (layer lines) and adding just as much to fill in the gaps.”

Before You Start the Smoothing Process

The following are a few tips to keep in mind before you start the process of smoothing your 3D print:

1. Fine shavings produced in filing processes and fumes generated by heat can be dangerous when inhaled. Most, if not all, smoothing methods require a well-ventilated area and/or a self-contained chamber to place your 3D print. Make sure you have the proper environment and equipment. Remember, safety first!
2. As mentioned, smoothing processes will involve removing or adding material. Keep in mind that once you remove material, you cannot get it back. If you are new to sanding or other methods that remove material, try practicing on some basic, throwaway prints before using them on pieces that you care about, or use an additive process instead. If you put on too much paint, for instance, you can always sand it back down.
3. As an extra precaution for larger prints, consider designing your assembly with multiple pieces that combine into the final piece. You can use specific smoothing methods for different parts, depending on what is most appropriate. It also gives you a little more flexibility if you mess something up during the smoothing process. Simply reprint the affected piece instead of having to reprint the entire project.

How to Smooth PLA 3D Prints

The following sections review different types of finishing processes for smoothing a 3D print, and best practices to follow.

Sand Down the Layers

Sanding is one of the most versatile, simple, and easy methods for finishing a PLA print. It is very commonly used to prep the print’s surface for covering techniques like painting or to enhance or smooth unwanted edges. To achieve the best smoothing results, you should at least pre-sand before using any other method.

The sanding method also works perfectly well on its own. With a little elbow grease, you can create a beautiful, smooth surface using different sandpaper grits. The best technique starts with using courser grades and progressing to finer grits. For example, start with 200 grit sandpaper followed by 400 > 600 > 800 > 1000 >1200 > 1500 > 2000 > 2500, and ending with 3000.

Though it does require a certain amount of effort, each layer should only take a few minutes, and the process gets easier with each level of grit. While you can attain pretty smooth surfaces with around 2000 grit, continuing the process all the way to 3000 grit will allow you to get an almost mirror-like finish.

Here are a few things to keep in mind while sanding:

• Sand in a circular motion, against the grain of the print’s layers.  
• Poor quality PLA can be hard to sand. If you are relying solely on sanding for a smooth finish, make sure you are printing with a high-quality PLA filament, to begin with.
• Never use power tools for sanding. Their vibrations and high amount of friction can create enough heat to melt or deform the plastic. Sanding by hand too quickly can also create excessive heat.
• Better yet, use wet-dry or waterproof sandpaper. As you work, you can wet the sandpaper to cool down the plastic and counteract the heat from friction. Wet-dry sandpaper is distinct for its black grit as oppose to tan or beige.  
• As you work, rinse your print often so you can see what you are working with more clearly. Rinsing also helps dislodge stuck particles and create a cleaner surface to work on. Moreover, it controls small, airborne particles that are harmful to inhale.

If your 3D object contains imperfections to start with, and you are worried that sanding might compromise the integrity, try using a filler to fill in any holes before sanding.

One of the drawbacks of the sanding method is that it does require some manual labor and a decent chunk of time. Large projects can be especially time-consuming, and objects with tiny details may be difficult to sand. The more time you spend sanding a piece, however, the more smooth the finish will be.

Carefully Apply Heat

Though you will hear over and over again that heat and 3D prints do not mix, there are actually certain applications for smoothing and finishing. While risky, a heat gun can be used to remove the visual layers from a 3D print. Since high levels of heat can melt plastic, the learning curve for this method is slightly higher than others.

Here are a few things to keep in mind while using a heat gun:

• Set the temperature of the heat gun to around 80 to 100 degrees Celsius, which is just slightly above PLA’s melting point.
• Keep the heat gun moving and never let it linger in one spot.
• Maintain a slow and steady motion to apply heat evenly.
• Practice on test pieces before using this method on a print that you care about.
• Work in a well-ventilated area as melting plastic produces toxic fumes.

Some claim that pre-sanding a piece can help; however, keep in mind that pre-sanded PLA takes less time to be affected by heat and starting the smoothing process.

Heat gun application is a quick smoothing method and may be particularly useful when smoothing numerous pieces. The Dewalt Heat Gun is Amazon’s Choice for heating guns.

If you do not have access to a heat gun, you can also use a butane torch at a very low temperature. Use the flame as you would a paintbrush to apply heat to the surface evenly. This method of flame polishing is actually quite common in acrylic plastic fabrication. Of course, such a technique requires considerable practice and patience.

Use a Chemical Solvent

Though acetone does not work on PLA prints, other chemical solvents do. Unfortunately, these chemicals are typically highly toxic and come with health risks, especially if improperly handled.

Solvents that work on PLA include:

• Ethyl acetate
• Pyridine
• Benzene
• Tetrahydrofuran (THF)
• Chloroform
• Acetonitrile

THF is one of the most effective solvents for smoothing PLA prints; however, it also happens to be excessively toxic. If you choose to use THF, make sure you practice safety first.

According to anecdotal reports, Ethyl acetate is safer, though ventilation and other safety measures are always a good idea. Ethyl acetate is highly flammable and can cause irritation to eyes and airways, disturb the central nervous system, and damage the lung in high concentrations.

Once you have set up your workspace correctly, simply add solvent to an old rag or other cloth that you won’t mind throwing away. Wipe your print with the rag dipped in solvent until the fabric is dry, then add more solvent. Repeat until your print is as smooth as desired.

Here are a few things to keep in mind while using chemical solvents:

• Always use heavy protective gear, such as Norfoil gloves and safety goggles.
• Your workspace should always be in a well-ventilated area. For vapor methods, a laboratory fume hood and respiratory gear are recommended.
• Chemical solvents are typically highly flammable and should not be used around heat sources. They also need to be stored carefully in a dry and dark space.

The advantage of the chemical solvent method is that it is quick and preserves the original color of your print. If you are working with decorative or colorful PLA filament, a chemical solvent is an effective way to enhance the original visual appeal.

For a chemical solvent to have the desired effect, the piece usually has to be relatively smooth to start with. If your print is on the rougher side, consider a light sanding before you get started with the chemicals.

Polysher

Polysher was created as an alternative to the acetone misting method used for prints. Many of the chemicals mentioned earlier are toxic and dangerous to use. If you are worried about safety but prefer a less hands-on approach than sanding or painting, Polysher could be the answer you are looking for.

With this method, your print is put inside a sealed chamber where isopropyl alcohol is atomized into a mist. The print then rotates in the mist, which reacts to the plastic, effectively melting it to create a smooth surface. The longer your print is allowed to sit within the chamber, the more seamless the surface will become.

Once the process is finished, your print will be sensitive to touch. To optimize the final appearance, let it sit for about half an hour before you handle it.

Cover the Layers Up

Sanding, heat, and solvent remove excess material to achieve a factory finish. There are plenty of other methods that do the opposite, adding to the surface of the print.

Some people prefer additive strategies because they provide some flexibility when it comes to mistakes, which is particularly beginner-friendly. For instance, if you add too much paint, you can sand some off. No harm, no foul.

Painting

Perhaps the oldest trick in the book, painting is an easy and stress-free way to cover up layer lines and create a smooth surface for your 3D print.

There are three basic steps for applying paint as a finish:

Prep

Before you get started with the painting process, you will want to make sure that the surface of your print is in pristine condition. Pre-sanding is an excellent way to eliminate protrusions and excess material.

After sanding, make sure you clean the surface with a brush and water to remove any trapped particles or dust. Before moving on to the next step, let your print dry completely.

Primer

Applying primer is necessary to hide any flaws on the surface of your print. You can use it to fill in ridges or lines and help level everything out. Make sure that the primer you purchase is compatible with your filament.

Spray on primers are quick to use and excellent at attaining full coverage. When dissolving the primer into a solvent, try not to shake the can. Instead, use a circular motion to avoid causing bubbles when the primer is sprayed.

After mixing the primer, spray on two coats, letting the first layer dry before you apply the next. You can also inspect the surface between coats to check for any additional blemishes that should be sanded down. Setting your print on a rotating platform can be useful to ensure that you spray the coats on evenly.

For finer details or touch-ups, you can use a brush on primer. Try to keep your final coat light to prevent the following layer of paint from bubbling.

Paint

Fortunately, many types of paint work well on PLA prints, including:

• Oils
• Enamels
• Lacquers
• Acrylics

If you are new to painting, consider using acrylics since they dry fast and can be cleaned up with water.

Paint can be applied with several methods, such as with a brush, airbrush, or spray can. Ordinary hand-brushing is cheap but also challenging to achieve even layers without noticeable drips or spotting. Airbrushing often results in a high-quality finish, but comes with upfront equipment costs and is difficult to learn. Spray paints are the perfect balance between the two.

Apply several coats to ensure full coverage, letting the paint dry between each application. Keep adding coats until you obtain the color you are looking for. To achieve a mirror-like surface, you can buff and polish the piece.

Here are a few things to keep in mind while using paints:

• Painting is best done indoors, protected from the outside elements.
• Your workroom should be well-ventilated. If available, use a respirator.
• Try not to spray too close to the print. Otherwise, you may get unwanted drips or an uneven coat.

To prevent unwanted cracks, use primer and paint from the same manufacturer and do not mix paint types.

A high fill primer spray can also be used on its own as a finish for your 3D print. Apply a thin coat, sand it down, so it fills in the visible layer lines, then fill in any and all imperfections. No paint necessary!

Polishing Compounds

Though designed to be used on metals, polishing compounds also work well enough on plastic and even glass. Just like sandpaper, they come in different grades from coarse to fine. You can rub a piece of clean cloth on the polishing compound bar and use the cloth to apply it to your print.

To speed up the process, use a polishing compound with a buffer wheel or similar attachment, and apply it to your print. The result will be a smooth and shiny buffed surface.

Just like sanding, using a polishing compound on a buffing wheel should be used with caution. The friction causes heat, which can melt or deform your plastic print. You should only use a slow speed and keep an eye on how hot the surface is getting. If the surface gets too hot, take breaks, and allow it to cool down.

Repeat application until the surface is perfectly smooth and shiny. Following, buff the surface by rubbing the print with a clean, unused piece of cloth.

Polishing compounds are typically sold as bars and can be found relatively cheap on Amazon.

Liquid Metal Polish

If you really want your print to shine, you can use a liquid metal polish on the surface. Use a soft cloth to apply a small amount of liquid polish onto your 3D print. Like with the sandpaper method, rub it in a circular motion against the grain. Make sure you really rub the polish into the surface so that it fills any micro-striations left behind.

For added shine, take an unused, clean cloth and buff the surface. This technique is often used to enhance results from sanding.  

Various types of liquid metal polish can be found on Amazon.

Epoxy

Epoxies consist of two liquid components that you will have to mix together: an unreacted epoxide and a curing agent. To create a resin-like material, one liquid catalyzes the other.

The ratio of substances changes depending on the type of epoxy that you use, so be sure to follow the instructions. If you are unsure about the correct amounts, take it drop by drop and engage in some trial and error. The ideal viscosity is a substance that is thick enough to fully cover your print, but thin enough to flow off your brush.

As an alternative to painting, the epoxy coats the surface of your print uniformly and does not leave any brush strokes.

Types of epoxy include:

• Smooth-on XTC-3D
• Z Poxy Finishing Resin

When working with epoxies, you have about 10 minutes before it sets. Curing times tend to vary between products. For instance, once you apply XTC-3D, you should let your print cure for about four hours. Other epoxies can even take up to 24 hours to cure.

After it cures, the resin will harden into a smooth, hard, and protective exterior surface. Some epoxies even have impact-resistant qualities that are perfect for projects like phone casings.

Epoxy is particularly glossy, giving you an attractive mirror-like finish. It also gives your print extra strength while preserving the original color and aesthetic.

It is worth noting that epoxies are not the best choice when it comes to prints that need to be safe for food and drinks. While some epoxies claim to be safe, there is still scientific uncertainty.

Sandblasting

Also called bead blasting, this method involves using a nozzle to spray the print with tiny plastic particles, resulting in a uniform, matte finish. Unlike other methods, the result is not shiny smooth and instead presents a textured surface.

Typically, the process takes place inside an enclosed chamber, which limits the size of prints that this method can be used on. Considering the equipment, it requires, this method is also pretty expensive compared to others.

You can use different types of media for sandblasting, such as thermoplastic particles and baking soda.

Air Eraser

A different, albeit less effective method of smoothing, is to use an air eraser kit. As a multi-purpose tool, an air eraser can delicately remove unwanted pieces of plastic. Though fast and useful for hard to reach places, an air eraser is not particularly strong and may be too weak to remove all imperfections.

Foolproof Combinations

As mentioned earlier, sometimes, the best techniques use a combination of methods. Try some of the following combinations to obtain the most polished finish possible:

• Filler followed by sanding, priming, and painting
• Sanding followed by heat gun application
• Sanding followed by epoxy and primer

Using multiple steps and methods tends to require a little more care and a lot more time, but once you see the quality of your finished print, it will all be worth it.

In Summary

As a biodegradable and environmentally friendly product, PLA is an excellent filament for your 3D printing needs. Unfortunately, PLA tends to have a rough finish compared to other options and requires additional smoothing to be aesthetically pleasing.

Since PLA does not react to acetone, you will have to rely on alternative methods to smooth your PLA 3D prints. Fortunately, there are plenty of methods to choose from. Sanding is the most basic method that is easy enough to learn and works well with other finishes. However, it is time-consuming and requires some manual labor.

For a quicker process, look to chemical solvents and careful heat application, though keep in mind the safety complications. If you want a little more control over your finish, try a covering method, such as painting, polishing, or using an epoxy.

When in doubt, try different methods on test prints before you use them on the final product. Experiment with combining strategies to achieve the best result possible. You might find that you prefer the matte finish of sandblasting as opposed to the high-gloss look that is common to most smoothing methods. 3D printing at home can be more than just an experimental hobby. If you take the extra time to smooth your PLA 3D prints, you can create truly professional-level prints that are worthy of display. You can even make a whole goody box of gifts come holiday season. Once you master your favorite smoothing method, your friends and family won’t’ even be able to tell that your presents are homemade 3D prints instead of store-bought.