When 3D printing an object, the lines between the various layers remain visible on the surface. They can hinder the aesthetic appeal of collectibles and decorative items.
This is where smoothing steps in – smoothing is a process that allows you to increase an item’s aesthetics, strength, and durability.
Smooth PLA prints are easy to achieve in a variety of ways. Smoothing techniques include sanding, resin coating, or employing chemical smoothing methods such as exposing the part to vapors of ethyl acetate.
Tumbling and sandblasting are two other ways to smooth PLA prints, but they are not as effective as others.
Acetone smoothing works for popular 3D printing materials like ABS, but not for PLA. Other methods should also be avoided by home users as they are too dangerous – even if they are effective.
How To Smooth PLA Prints
Safe PLA smoothing methods range from sanding to chemical smoothing and heat treatments.
Pick your favorite method from the list below to improve the quality of your PLA items.
1. Sanding
Sanding is the easiest and, often, the most accessible way to smooth PLA. This method suits beginners and those who don’t want to use solvents for one reason or another. Its only downside is the length of time it takes to achieve a satisfactory finish.
For effective PLA smoothing through sanding, you must sand the surface with coarse to ultra-fine sandpaper.
Start with 100-grit and shave off all excess material. Then, switch to 200-grit sandpaper and start smoothing out the print lines and other surface defects.
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Work your way up to 1,500-grit sandpaper. For the finest grit, you can wet the sandpaper first for more effective smoothing. Final polishing can improve the results even more.
2. Polishing
Polishing the object, usually with a plastic polish, is another simple and effective method of removing printing surface defects.
Polyurethane sprays adhere to PLA surfaces without the need for a primer. However, coating the surface with a plastic primer could improve the results and increase the coat’s durability.
You can polish the object directly or sand it first, depending on the intensity of surface defects.
Simply follow the instructions on the polyurethane spray to prepare and treat the material properly. Let the spray dry and cure as instructed.
An alternative to polyurethane spray is 3D Gloop. This is an adhesive designed to improve the adherence of the build plate, but it works well for PLA smoothing too.
Simply coat the object with a thin layer of adhesive and let the glue dry. The results are satisfactory, but the color might be altered.
3. Surface Coating
Surface coating is similar to polishing, but the method involves the use of epoxy resin. No prior preparation is necessary other than making sure the surface is clean.
In case of major defects – such as elephant’s foot, zits, stringing, or gaps in the layers – you might want to treat them first through sending or filling in the gaps with a gap filler.
A good resin product is the XTC-3D, a two-part coat formulated specifically to smooth PLA prints.
The main difference between this product and other smoothing resins is the low amount of heat it generates while reacting with the material.
Since PLA has a low glass transition temperature and melting point, this factor is essential to avoid damaging your prints.
Like most resins, XTC-3D requires you to mix the resin part with a hardener. Thus, you should only mix small quantities at a time, or the excess resin will become unusable.
Once you added the hardening agent, spread it onto the printed item with a paintbrush. Work on small portions at a time and make sure the surface is covered properly before moving to the next patch.
Let the resin harden and cure as instructed before using the part.
Surface coating is particularly effective for miniatures, collectibles, and other decorative objects.
4. Solvents
PLA and solvents don’t pair well, but there are a few that you can use. Among readily-available chemicals, 3D printing enthusiasts can use ethyl acetate.
This chemical can be found in numerous solvents for domestic use, including nail polish removers.
However, for the best results, you should use a concentrated product, such as lab or technical-grade –you can find these products in most home improvement and tech equipment stores.
To smooth the print, you can soak a soft cloth into the solvent and dab the print to soften and smooth out defects.
Other lab-grade chemicals also work, but we wouldn’t recommend them to home users. They are very dangerous to work with and simply not worth the hazard.
Sadly, there is no other PLA smoothing solvent that you can use safely in a non-industrial setting.
5. Vapor Smoothing
Vapor smoothing is another chemical process achieved with solvents. However, instead of dabbing the chemical directly onto the part, you place the item in a closed container and let the chemical vapors smooth it.
This method is typically effective for smaller items that can fit into a Tupperware container or closed bucket.
Coat the walls and bottom of the container with paper towels soaked in ethyl acetate. Place a support inside the container and position your part on it.
Close with an airtight lid and let the solvent vapors smooth out the part for the next few hours. Check the progress constantly to avoid damaging the part due to prolonged exposure to solvents.
6. Heat
Another effective method of smoothing PLA is with heat. There are two ways to achieve this – by melting the superficial layer with a heat gun and smoothing the material, or through annealing in a gas or electric oven.
Heating the surface with a heat gun is as straightforward as possible, but also risky and potentially dangerous.
Set the heat gun at the lowest setting and heat the part uniformly (preferably by placing it on a turntable).
Hold the gun at least 10 inches from the part to prevent overheating. Too much heat can melt the entire surface of your object, ruining it completely.
Alternatively, heat the part in an oven.
If you have an electric oven, fill a container made of oven-safe glass with sand and bury the part in it. Meanwhile, preheat the oven to 70°C (158°F).
Wait for at least 30 minutes to make sure the temperature is constant, then place the container inside the oven.
Cure the parts inside the oven for at least one hour, then remove the container and let it cool down to room temperature. Only remove the parts from the sand after cooling.
If you’re curing more than one piece, make sure there is a space of at least two inches between the parts and from the container walls and bottom.
Annealing is possible in the gas oven too, but the procedure is slightly different due to the inability of gas ovens to maintain heat perfectly stable.
As above, preheat the oven to 70°C. Place the parts in an empty tray (or any oven-safe surface) and place them inside the oven. Turn off the heat immediately.
Leave the prints inside the oven until the unit has cooled down completely. You may have to repeat the steps a few times, since annealing in the gas oven is not as effective as annealing in an electric oven.
3 Tips To Smooth Out PLA 3D Prints
Knowing what methods work for smoothing PLA might not be enough if you don’t know how to use them efficiently.
Regardless of which method you prefer, the tips below might help you improve the results.
1. Remove Excess Material Before Smoothing
No matter which smoothing method you prefer, removing excess material before starting can drastically improve results while cutting down work.
A straightforward way to remove excess material is with a utility knife. If you don’t have the tool already, the most important thing to check before buying is the blade.
Make sure it is thin but sharp and sturdy. A thin blade is recommended to avoid trimming the model excessively.
A utility knife can help you remove brims, rafts, and other support types. Always start from an edge of the model and cut in a straight line (following the model’s contour) moving the blade from near your body and away from yourself.
Avoid applying too much pressure as you’re trimming, or you could damage the model.
If you don’t have a utility knife and don’t want to buy one, you can achieve the same result with a sharp cutter or razor blade.
Another way to remove excess material is with a rotary multitool. Dremel is a fine example of such a tool, but there are similar brands out there.
Dremel tools come with abrasive attachments that work more like sandpaper than a knife. However, you can achieve the same result with little effort.
Choose the bit that’s most appropriate for your model shape and use it to sand down excess material. Brush the dust off after finishing, especially if you want to smooth the model with a chemical agent.
While Dremel tools are ideal for removing excess material, the bits are usually too coarse for proper sanding. Using a sander isn’t recommended either, as you risk removing too much material.
2. Use a Primer
Using a primer is particularly effective when the part presents both over- and under-extrusion defects.
These could be alternations of zits and holes, and in this case, a pre-treatment with the Dremel tool or a utility knife plus a coat of primer can improve results.
Primer not only improves the adherence of resins and paint products, but it also fills in the gaps – as long as they are tiny ones.
Most plastic primers work great with polylactic acid, including Rust-Oleum 2-in-1 primer and filler. Similar products work well too, so you can use whichever is available at your home improvement store.
3. Paint Instead of Smoothing
If the part only has small defects and want to cut down processing times, you could skip smoothing completely and paint it.
While this method requires no sanding, you might want to trim down excessive material and remove burrs with fine-grit sandpaper.
Once that’s done, find a sheltered spot in a well-ventilated area and paint the part with your favorite plastic paint. Spray paint is the best choice for application ease, but you can use whatever color you want.
Coat the paint with a thin layer of clear polyurethane right after painting, before it dries. This will allow the polyurethane and paint to blend, covering imperfections and giving your part a smooth and polished finish.
What NOT To Use
If you’re new to 3D printing with PLA but have used other materials before, such as ABS, you may be tempted to use similar methods to smooth PLA.
ABS smoothing methods often involve the use of acetone, which works well for vapor smoothing. Harsh substances are also effective for ABS.
However, they don’t have the same effects on PLA. Here are a few things you should never use for PLA smoothing.
Acetone
Acetone-smoothing PLA is something that will never work, due to the effects this solvent has on polylactic acid.
When the two get in contact, acetone alters the chemical bonds of PLA, turning it into a rubbery, sticky substance.
Depending on how prolonged contact is, you may end up with a slightly sticky surface (acetone acts as a glue in this case), or with a completely destroyed part that crumbles to bits in front of your eyes.
Bottom line: don’t use acetone to smooth PLA, or you’ll end up regretting it.
Harsh Chemicals
Advice online has people swearing by smoothing PLA with harsh chemicals such as chloroform, tetrahydrofuran, or dichloromethane.
Sodium hydroxide is another common contender, but you should never use any of these substances – unless you can do so in a controlled environment, such as an industrial lab.
First and foremost, chloroform and company are not chemicals that you can buy at the local chemist’s shop. The only way to get them is from an industry allowed to use them legally.
They are very dangerous to human health and could lead to injury and death if handled incorrectly. Moreover, there is no proof whatsoever – apart from anecdotal evidence – that these chemicals work for the intended purpose.
Sodium hydroxide – aka, lye – is readily available on the market. This is not a controlled substance, but it is a harsh base that can react with the acidic pH of your skin, generating a thermogenic reaction that can burn right through your skin and flesh.
Lye, too, can cause severe injuries and even death when handled incorrectly. The substance is also more likely to disintegrate PLA into small bits rather than smooth it.
Conclusion
The safest way to smooth PLA is by sanding and polishing it, even though the process is tedious. Ethyl acetate acts like acetone. Using heat, especially the annealing process – can smooth your PLA parts, too.
Acetone and harsh chemicals may be recommended in some forums, but they don’t work on PLA. These substances are also dangerous, and you should stay away from them.
