How to Smooth PLA 3D Prints

In this instructable I am going to show you how I smooth PLA 3D prints with four different methods.

Thanks to instructables in 2013 my life as a maker changed a lot. In that year I won my first 3D printer with my Nerf Sentry Gun instructable. You can't imagine how nervous I was, waiting for the winners to be announced. A 3D printer was something I wanted more than anything else, but I couldn't afford one (at that time the Makerbot Replicator was $2000).

If you take a look at my ibles from the last years you can tell that 3D printing is one of my favourite things ever. I have spend thousands of hours seeing my ideas come to life and I am very sure that I will never get tired of it.

Since my enthusiasm is apperently infectious a lot of my friends have bought 3D printers or ask me to print things for them (which I love doing). Now the question I get asked the most is: "How do you get your prints to look so smooth?".

I (like most people) print mainly in PLA and while ABS is quite easy to smooth, PLA is a bit more tricky. So eventhough there is nearly always a step on my instructables to show you how I finished and painted a project I decided to write this instructable to collect all the essential information in one place and hopefully help you safe some time.

There isn't just one perfect method to smooth PLA since a lot of things have to be considered. For example the size of your model, the amount of details it has, whether there are hard to reach areas, the preservation of the original colour or how much time you would like to spend.

My four favourite methods to smooth PLA 3D prints are shown in the table below.

There are also other methods like using a heat gun, but since those didn't work that well for me, I didn't include them in the list. You can obviously just sand and polish (e.g. with metal polish) the part, but it will take a lot of effort and is tricky if your print is complex.

If you have a method you think works well, please tell me, I am always looking for improvement.

Here, here and here are three more instructables by other members that you might find helpful.

To show you how the different methods work I decided to go with everybody's favourite torture test: 3DBenchy.

It was printed at a layer height of 0.2 mm (I usually go with 0.15 or 0.2 mm for big parts). Since it is a rather small part I would usually go with solvent smoothing, but decided to show you every process with the same model. All 3DBenchys were printed with my Ultimaker 2+ extended and I used Simplify3D as a slicer.

I have also included some images of fails I had in the past, so that you can hopefully learn from them.

Here are a few of my project that I smoothed after printing, should you be interested:

Deadpool Knife Block I started by using primer, but it didn't properly fill the holes, so cracks started to form. I then covered it in epoxy used another layer of primer and repainted it.

Thor's Hammer Screwdriver Kit I used epoxy for the handle and wiped the runes with THF. In a second build of this project I used an air eraser to smooth the runes.

Minion Fart and Smoke Alarm Once again I used epoxy and this time I sanded the pieces.

Double Pendulum Fidget Spinner I used THF to smooth the parts.

Qi Green Lantern Charger I decided to look into why I had problems when trying to smooth the Deadpool and gave using filler primer and sanding another shot.

There are a few more on my instructables page, feel free to check them out. As you can see I use different methods for different projects. Sometimes I even use different methods on one project.

Before you start, remove all the support structures and if your model consist of multiple parts, glue them together.

Use a scalpel to remove all small imperfections, like oozing.

Now start filling bigger imperfections and holes. I use autobody filler (e.g. from amazon). I have read that people like using wood filler. Personally I prefer using autobody filler, because in my experience it sticks better to PLA and since it consists of two components you can use thicker layers. Also, it doesn't shrink much, which means you are usually done in one go.

Afterwards you will have to sand the areas you just filled. At this stage you should be fine with a 320 grit. Never use an electrical sander, since the piece might get hot and the PLA will start melting.

Tips:

Make sure to fill bigger holes with body filler first, don't try to use just primer, especially not with one thick layer. As you can see in the picture, it will result in indents and sometimes even cracks. Due to the thick layer, the primer won't dry on the inside and it will start to shrink after some time.

Another thing that has happened to me was that the glue I used reacted with the primer and small bubbles started to form. So before you start I suggest putting a drop of glue somewhere and spraying it with the primer to see if they react.

It is always better to hold the spray can upright. So unless my model is really big I use blu tack and stick it to a handle (of an old hammer) so that I can easily spray paint from all sides.

The most important thing to keep in mind is to spray in thin layers. To safe some time use filler primer (e.g. this one). Let it dry properly, before spray painting another layer onto it. I usually use two to three layers before sanding the first time.

Instead of using primer filler you can also use Plastic Putty.

Make sure that your work area is clean, well ventilated and read the instructions on your spray can so that you know at what temperature ranges it works best. Depending on how smooth you want your model to be you might not even have to sand the piece, since filler primer already works really well.

Tips:

One of the best tricks I found to get a nice, even and smooth result was to wet sand. You will have to get the right type of sanding paper (e.g. this collection). Simply leave it in water for about 15 minutes to get it soaked and go over the part with it. I have been told that it works even better if you leave the sanding paper in the water over night, but so far I haven't found it necessary. Make sure that your sanding paper stays moist and that dust doesn't start clogging up, by simply dipping it into water from time to time.

Adding a little bit of soap to the water helps to lubricate the surface, prevents your sanding paper from clogging up and helps to enable particles to rinse more freely off the surface.

I usually fold the paper over on itself to create a thick piece to hold on to. You can obviously also use a sanding sponge.

At this stage I usually use 400 or 600 grit sand paper. I was taught that you should never sand through the primer, especially if it is applied to wood, but I found that with PLA it doesn't really matter (unless you don't want to use another primer coat afterwards). Once you are done sanding, wash the model and let it dry (don't use a blow dryer unless you are very careful). Now you can decide, whether you would like to go with another layer of filler primer and sand again or use another primer with a thinner consistency before you start painting. I love Army Painter Base Primer (e.g. from amazon), since it comes in different colours, sprays nice and even and works really well with acrylic paints. In my experience Black Primer Formula P3 (e.g. here) is also very good.

Back in 2013 when I started trying to find a way to smooth PLA 3D prints there was not much information around. Since I didn't want to blindly trust the few online recommendations I could find, I decided to do some research and found this paper. It compares the solubility of PLA in different chemicals and also gives information whether using a certain chemical would result in swelling.

Overall the research says that quite a few solvents would work. E.g. Pyridine, Benzene, Ethylacetate, Tetrahydrofuran, Dichloromethane, Chloroform or Acetonitrile.

Since Tetrahydrofuran (THF) was easy for me to get, its relatively low boiling point, and compared to other chemicals low hazardousness (at that time, please take a look at the next steps to see what I mean with that) I decided to go with it.

Please bear in mind that different PLA filament brands might behave different, since you will never get filament that is pure PLA. Usually there are a lot of additives added to it, like for example softener.

For a long time THF was my favourite way to smooth PLA 3D prints. But since in 2017 it was classified as "carcinogen in laboratory animals" (source), I decided to use it less and look for alternatives. As you can see on my bottle in the picture above, at the time I got it, it wasn't labelled as a "GHS08: Health hazard".

After the Globally Harmonized System of Classification and Labelling of Chemicals it is classified as "H351: Suspected of causing cancer". Like all other solvents that would work it is also classified as "H225: Highly flammable liquid and vapour" and irritating.

Should you be interested here is a list of all the physical and health hazards (find out what they mean here):
H: 225‐302‐319‐335‐351
P: 210‐280‐301+312+330‐305+351+338‐370+378‐403+235

Make sure to read through all the hazards and proceed at your own risk. The GESTIS Substance database is a very good source to find out more about chemicals.

Since THF is such a great solvent, most gloves are useless and it will eat right through them. Norfoil gloves are recommended, but they are hard to come by. Polyvinyl acetate (PVA) gives some limited protection (up to 1 - 1/2 hours for some gloves) (source). I use PVA gloves when handling THF. Also make sure to wear safety goggles.

Another thing you have to be aware off when working with THF is storage. As EvilChemist points out in the comments in contact with light and air, explosive peroxides might form. Make sure to store it in a closed brown glass bottle and keep it away from light. If you are worried about peroxides you might consider buying peroxide testing strips. Do not evaporate or distill it (especially not in a closed system) unless you have tested it.

Make sure to work in a well ventilated area and that there is nothing nearby that might cause a fire.

Overall as you can see above, you will have to decide whether a smooth part is worth the risk.

As stated in the last step, after THF was classified as being suspected of causing cancer I started to look for an alternative.

I've seen a few webpages that state that chloroform works really well to smooth PLA but since it is illegal to buy as a private person in most countries and it is like THF classified as "H351: Suspected of causing cancer" and furthermore as "H331: Toxic if inhaled", I advise against using it.

So I ended up starting to use ethyl acetate and found that it works really well. Ethyl acetate is commonly used in glue and nail polish remover. That's why in other instructions nail polish was found to work well.

Like THF, ethyl acetate is a highly flammable liquid. It also causes slight irritations to the eyes and airways, shows disturbance to the central nervous system (narcotic effect) and damages the lung in high concentrations. So make sure that you have a very good ventilation in the working area. Vapour/air mixtures are heavier than air. Adequate ventilation at the floor area must be ensured as well.

Use protective gloves. I use butyl rubber ones. It will eat right through latex or nitrile gloves, so those aren't sufficient. Also wear eye protection.

There are two different ways to smooth with solvents.

1. Use a cloth and wipe the part
2. Use vapours

Personally, most of the time I simply use a cloth and wipe the part with it. I often do it, even if I plan on using a primer later on.

As you can see in the picture the PLA will start to stick to the cloth, so use a cheap one you wouldn't mind throwing away (e.g. these). You can use a Q-tip for the harder to reach areas.

You will have to use some pressure to get a nice and even result. Simply add a small amount of solvent to the cloth and wipe the part with it. Once the cloth gets dry add more solvent and start over. Using solvents is faster than priming and sanding, but the part has to be rather smooth from the beginning for it to work well.

So far I wasn't successful in vapour smoothing PLA with ethyl acetate, but I decided to share my findings here.

Ethyl acetate has a vapour pressure of 9.7 kPa at 20°C which means that at 20°C (68°F) about 10% of the gas phase consists of ethyl acetate. You can see a visualisation of the vapour pressure in the first image. My shop temperature was at about 15°C during testing and even leaving the print for hours in the vapour didn't have an effect. If you use acetone (which works great for ABS, but sadly not for most PLA brands) at 20°C already about 25% of the gas phase consist of it.

In order to achieve a higher concentration you will either have to increase the temperature or decrease the pressure. As you can see in the second image I decided to start with increasing the temperature.

I tried different temperatures in between 40°C (104°F) to 60°C (140°F) and left the piece in the vapour for up to two hours. Unfortunately, nothing worked. As you can see in the third picture the bottom the benchy started to deform before the top started to smooth.

The next thing I tried was reducing the pressure. In this case, the ethyl acetate started to dissolve the container (you can see it turning white in the fourth picture), even though it was supposedly made out of polyethylene. At this point I didn't experiment any further, but if you do own a better vacuum chamber please tell me about you findings.

I never thought about using epoxy, until XTC 3D was started being sold. So I decided to give it a try. I enjoyed working with it, but sadly it is really expensive in Germany. I experimented a bit further and found that any other epoxy will work just as well.

Read all the safety instructions before working with epoxy! The hardener of XTC-3D is for example labeled as "H361: Suspected of damaging fertility or the unborn child". Which is an other reason why I am using a different brand by now.

When working with epoxy make sure to wear nitrile gloves! Hazardous substances like ethylenoxide can diffuse through latex gloves and you definitely don't want those on your skin! Also make sure to wear safety goggles.

Epoxy is made up of two parts, an unreacted epoxide and a hardening or curing agent. You will have to mix the two components. The correct ratio depends on the epoxy you use, so make sure to read the instructions. The curing time also differs on the epoxy you use. No matter which epoxy always make sure to mix thoroughly.

Paint brushes can be cleaned with acetone, but it is quite messy, so I suggest using disposable brushes. I like using foam brushes (e.g. these, if you buy them in bigger bags they get even cheaper). Sadly in my experience Q-tips don't work really well. After a bit of try and error I found out that disposable lip brushes work really well (e.g. these).

All you have to do is carefully paint the epoxy in a thin layer onto the 3D print. Like with spray painting the part, I stick it to a handle with blu tack.

Let the epoxy cure for at least 24 hours. Once it is dried you can either leave it as is, sand it, or apply another layer of epoxy. The reaction is exothermic, which means that heat is generated. This happens especially in your mixing cup. Therefore do not use a very narrow and high container or it might result in a runaway reaction. This means also that due to the heat generated the epoxy in your cup will cure faster than the one on your model. So please don't touch the model, just because the epoxy is cured in your cup.

Personally, I am not a big fan of sanding epoxy. If I really want the perfect finish and since I usually always paint my parts, I usually apply one layer of epoxy, then let it dry and use filler primer as a second layer.

Tips:

You might notice that the epoxy you are planning on using is too thin. This can easily be fixed with thixotropy powder (read more about it here). Simply add a small amount to your epoxy, stir properly and add more if needed. It will turn the epoxy slightly white, but since a very thin layer will be applied later on it doesn't matter and can't really be seen. As you can see in the picture XTC-3D is also quite white.

Make sure to use thin layers. Otherwise drips might form that are hard to remove.

To see whether the Deadpool Knife Block was really smooth I decided to spray paint a layer of primer onto it. After seeing that it wasn't I applied another layer of epoxy and the result was horrible. As you can see in the image on the right, the epoxy formed a really weird, absolutely not smooth surface. So never apply another layer of epoxy to a spray painted surface unless you are very sure it will work.

After seeing this instructable and already owning a sand blaster I decided to give it a shot. For sandblasting I am using a mixture of silicon dioxide and aluminium oxide as a blasting abrasive with a grain size of 0.2 to 0.5 mm and a pressure of 8 bar (116 psi).

I honestly rarely use sandblasting to smooth 3D prints, because even though it works it mainly just makes the part look smoother due to becoming matte, but if you spray paint it afterwards you can see that lines are still rather visible. Also, as you can see if you have a lighter part, you shouldn't use the same mixture I do, since it stains the piece.

It works perfectly though if you have a part with deep indents as the one shown in the third picture.