Smoothing 3D Prints: How To Smooth PLA to a Mirror Finish
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5 common ways of smoothing 3D prints include:
- Epoxy coating
- Heat gun
- Chemical smoothing
Layer smoothing refers to a set of different post-processing techniques that help smooth out layer lines on 3D prints.
You can use many layer smoothing techniques with PLA prints. Sanding is the easiest but takes the longest, and there are other options like using a primer, epoxy coating, or chemical smoothing.
Because of this, it’s no surprise that layer smoothing is common for 3D printing cosplay and other display-type models.
- How to Smooth PLA 3D Prints
How to Smooth PLA 3D Prints
Smoothing PLA 3D prints involves using compatible layer smoothing techniques to give your PLA 3D prints a glossy and mirror-like surface finish.
However, not all layer smoothing techniques, such as acetone smoothing, work with PLA plastic due to its chemical and mechanical properties.
In the sections below, I’ve gone over the most effective techniques for smooth PLA prints, that you can use for perfect finished prints!
Sanding works on every filament material, from PLA to PC and everything in between. Simply rub a piece of textured sandpaper along the outer surface of a 3D print, and over time the PLA’s surface will match the texture of the sandpaper, allowing you to control the “grit” of your print.
Sanding is commonly the first layer smoothing technique people use as it smooths out the larger layer line separations. Plus, it’s easy and cheap. But, it can be time consuming, especially for larger models, as you can only sand a small area at a time.
How to do it:
From my experience, the best way to sand your PLA 3D prints is to:
- Start with a low-grit (rougher) piece of sandpaper, like 80 or 100-grit.
- Then, move up to 150 or 200-grit sandpaper.
- Finish the surface off with some 300-400-grit sandpaper.
This three-stage sanding process will give your PLA print a very smooth surface finish.
I also suggest dipping your sandpaper in water after each scrub (“wet sanding”), as this helps lubricate the sandpaper. Wet sandpaper is faster and speeds up the sanding process.
You should also wash your 3D print in water and dry it off with a towel or cloth after you’re finished sanding, to remove any unwanted plastic bits on the model’s surface.
We have also written a more in-depth, step-by-step guide to sanding PLA if you want the full details.
Then once fully sanded, you can get shiny, polished models that look like this 3DBenchy I worked on:
Primer is a paint-like substance that helps fill in any small gaps, cracks, or crevices on the surface of a part. It’s generally used before applying your actual coats of paint to finish the 3D print, and is ideal for smoothing PLA layer lines.
Filler primer adds to the surface to smooth layers, whereas sanding removes the surface. Primer is much faster than sanding (it only takes around 10 minutes), as you just spray or brush it over the part’s surface.
How to do it:
To apply your spray primer, start by placing your PLA 3D print on some newspaper or cardboard outside or in a well-ventilated area.
Next, shake the bottle and spray it from around 3-5 feet back from your model. Start spray painting the PLA by moving the spray can nozzle in swift horizontal motions until your entire model is coated in the primer.
After letting the thin layer coat dry for 10-30 minutes, I suggest applying a second spray paint coat of the primer to ensure the surface of your PLA print is as smooth as possible.
The best primers for PLA 3D prints are those that are specifically compatible with plastics, including acrylic-based primers and many universal primers. My personal favorite option is Rustoleum’s Painter’s Touch 2X Ultra Cover primer, as it’s very inexpensive and works wonders on PLA 3D prints.
3. Epoxy Coating
Epoxy is a type of resin material that hardens when mixed with the right chemical.
When you mix epoxy resin with the curing agent and apply it to the PLA print’s surface, the gel-like substance bonds to the part’s surface, going over the original rough surface of the part. Once dried, your model will have an ultra-smooth, glossy surface.
Because you have to mix chemicals together, and in the right proportions, epoxy coating is a relatively difficult method for PLA smoothing.
But, as I mentioned, the surface finishes that epoxy coating provides are so immaculate that it’s still worth it if you want the most visually-appealing model possible.
However, note that epoxy resin is known to weaken the PLA material beneath the outer coat. So, you might not want to use this method of layer smoothing if your PLA part is going to be under physical stress. It’s always better to be safe than sorry!
How to do it:
To use epoxy to smooth your PLA part, start by mixing together the epoxy resin and the curing chemical.
I recommend purchasing an epoxy resin set that includes both of these substances so you get the right amount of each. The J-B Weld Epoxy kit on Amazon is a terrific option that’s also super affordable (around $10).
Then, use a paintbrush to coat the entire surface of your model in the mixed epoxy resin. I recommend brushing each section of your model a few times to achieve the best results.
Once you’re done painting the substance, let it dry for at least 30 minutes before touching the model. Some epoxy resins state that they dry in 5 minutes or less, but more drying time is always safer.
Once finished, I suggest rinsing your model in water to clear away any leftover epoxy residue.
4. Heat Gun
A heat gun is like a super-powered version of a hair dryer, heating up the air with metal coils and blowing hot air out of a cylindrical nozzle.
You can use a heat gun to smooth your PLA 3D prints by lightly melting the outer layers of the model to blend the layer lines and remove inconsistencies like stringing and blobs. This is possible due to PLA’s low glass transition temperature of around 60°C.
How to do it:
Start by setting the glue gun’s temperature to the lowest mode, and wait for the air to heat up.
Then, point the nozzle of the heat gun towards a section of your PLA 3D print and leave it there for 1-2 minutes. You should start to see the layer lines smoothening out, but, if not, wait a few more minutes or increase the temperature.
Then, repeat this across the entire surface of your PLA print, until it’s all smooth! I personally use a turntable to rotate the 3D print as I heat it to achieve an even surface finish, but this isn’t necessary.
I highly recommend this method of layer smoothing because it’s safe, easy, and very affordable. On that note, you can typically buy a heat gun for $15 and $40 online or at a local hardware store.
5. Chemical Smoothing
Chemical smoothing is undoubtedly the most effective layer smoothing method on this list, and creates the smoothest surface finishes. This method involves using a solvent chemical to lightly dissolve the outer layers of the PLA to achieve a super smooth, mirror-like finish.
Different filaments are soluble in different chemicals, so while most filament materials can be chemically smoothed, they require different solvents.
Compatible PLA solvents include special formula products like 3D Gloop and XTC3D (technically an epoxy), as well as generic substances like ethyl acetate, methyl ethyl ketone (MEK), and tetrahydrofuran. (Ethyl acetate makes up nail polish remover, which you may have heard to work well with smoothing PLA prints by literally dissolving PLA.)
The generic chemical options are much cheaper than the proprietary PLA solvents, but they also might be more difficult to find. You don’t tend to find ethyl acetate and MEK in the check-out aisle at a local grocery store!
However, if you’re looking to achieve the smoothest surface finish possible, I suggest using one of the proprietary options, like 3D Gloop. They’re designed specifically for smoothing PLA 3D prints, and they’re very effective.
How to do it:
While the chemical smoothing process for PLA varies depending on what solvent you use, the general process involves applying the solvent to the outer surface of your 3D print, and letting it dissolve the layers.
This process can take anywhere from 1 minute to an hour, depending on the substance you use.
After the solvent dissolves the layers and gives your model a super smooth finish, make sure to rinse the model in water to remove any leftover solvent residues, as they are usually toxic.
Safety Precautions To Remember
Solvents like ethyl acetate and other solvents are health hazards, and it’s essential to take the right precautions when handling them.
- Ventilation: Always work in a well-ventilated area when using solvents. The fumes from these chemicals can be harmful if inhaled in large quantities. They may cause dizziness, headaches, and longer-term health issues with sustained exposure. If possible, perform the work outside or in a space with a good ventilation system.
- Eye Protection and Skin Protection: Wear safety goggles and gloves. Solvents can irritate or burn your skin and eyes. Avoid direct contact as much as possible.
- Clothing: Wear long sleeves and pants to minimize skin exposure. Some chemicals can cause skin irritation or burns. It’s also a good idea to wear an apron to protect your clothing, and nitrile gloves to protect your hands.
- Fire Safety: Many solvents are flammable. Keep them away from open flames or other fire hazards.
- Storage: Store chemicals properly when they’re not in use. They should be kept in a cool, dry place with adequate ventilation. The storage area should be out of reach for children and pets.
- Disposal: Dispose of any waste properly. Don’t pour leftover solvents or other chemicals down the drain. They can harm your plumbing and the environment.
What solvent should you use for PLA smoothing?
You can use either proprietary or generic solvents to chemically smooth your PLA 3D print. I strongly recommend going with a proprietary solvent, such as XTC3D or 3D Gloop, as these solvents are much easier to use than generic chemical solvents, like ethyl acetate or tetrahydrofuran.
Plus, XTC3D and 3D Gloop both create a mirror-like finish when applied correctly. While XTC3D and 3D Gloop cost a bit more than generic options, the results are definitely worth it!
Can you use isopropyl alcohol to smooth PLA?
Typically, the answer is no. Most PLA 3D printing filaments are not soluble in isopropyl alcohol (IPA), so it’s not a compatible solvent option.
However, certain PLA-like filaments, such as Polymaker PolySmooth or Prusa’s PVB filament, are soluble in isopropyl alcohol. But that’s only because these materials aren’t actually pure PLA and contain additives like PVB that make the filament soluble in IPA.
As for what isopropyl alcohol to use, any bottle of 100% pure IPA should work perfectly. You might also be interested in Polymaker’s Polysher device, a containment chamber that evenly sprays your model with IPA vapors to smooth out the surface.
What is the easiest way to smooth PLA?
The easiest way to smooth PLA is sanding. All you have to do to sand a part is grind a piece of sandpaper back and forth against the surface of a 3D print. It doesn’t get much easier than that! Just scrub and rub!
While it’s recommended that you sand your 3D print in a few stages, starting with low-grit sandpaper and moving up, you don’t have to.
Though, while it is the easiest, sanding is not the most effective way to smooth PLA. Other options, such as chemical smoothing, yield much smoother surfaces than sanding.
Can PLA be Acetone Smoothed?
Most regular PLA filaments cannot be acetone smoothed as filaments like ABS can. This is because PLA in its pure form is not reactive to acetone.
When PLA and acetone smoothing works, it’s due to other materials added to the PLA. Sometimes, you can acetone smooth very cheap PLAs that have been blended with cheaper plastics to form a less pure PLA blend.
Generally, the purer the PLA, the less acetone has an effect on it. If you want to acetone vapor smooth 3D printed parts, you can read our article on acetone smoothing ABS and ASA.
Sometimes, PLA vapor smoothing can be achieved with Ethyl Acetate or even Tetrahydrofuran (THF), but I strongly advise against using either of these chemicals. They’re dangerous, the effect only works with some PLAs, and you can lose a lot of detail.
Additionally, this technique to smooth 3D prints (with PLA filament) does not work as well on very intricate prints.
Here’s a close-up of the finished effect. The gold filament is Gold/Bronze PLA. This is a 6cm long 3D Benchy for scale, and we only smoothed one side of the hull to compare.