How to Smooth PLA to a Mirror Finish

Need to make your prints look super smooth? Or perhaps you just want rid of those layer lines to make your print look more 'factory finished'? This is one of our favourite PLA finishing techniques: how to polish PLA 3D prints to a shine you won't believe was ever 3D printed. 

The best way to smooth 3D prints.

It's often said that ABS is easy to smooth (using acetone baths and similar methods), but that PLA and similar non-acetone-dissolving 3D printer filaments cannot be smoothed the same way. They can - you just need to know the right technique, and have a little time on your hands. 

Sometimes, PLA vapour smoothing can be achieved with Ethyl Acetate or even Tetrahydrofuran (THF), but we 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. 

We recommend smoothing using the technique we're describing here for your most prized prints, as it can take a little bit of time. 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 our Gold/Bronze PLA. This is a 6cm long 3D Benchy for scale, and we only smoothed one side of the hull to compare. 


Polished 3D Benchy

(it's hard to see just how smooth this surface is, but honestly, it's like glass)


What you'll need:

To start, you're going to need a selection of fine grit sandpapers. We suggest the following grit sizes (and if you want a really smooth finish like above, we recommend not skipping any steps): 200, 400, 600, 800, 1000, 1200, 1500, 2000, 2500 and finally 3000. 

It's best if you can use 'wet & dry' paper (black grit as opposed to beige effect), especially with PLA, because if you're sanding fast, heat can build up and affect the smooth surface you're trying to achieve. Wetting the glass-paper will lubricate and cool the surface when you're sanding PLA prints. This is not as necessary with smoothing higher temperature 3D printing materials, but still advisable. 


1. Start with the coarsest sandpaper (approx. 200), sand and smooth the surface until you cannot feel any printing layer lines at all. You've got to sand in a circular motion, as you don't want any lines from the sandpaper affecting the surface later on. 


400 Grit Sandpaper 3D Print

 (this is what your print will look like after using 400 grit sandpaper; you cannot see the layer lines)


 2. Start scaling up the grit number. Once you've got an acceptable result with the 200 or 400, move higher up and smooth the print in increments. Don't skimp on the time to sand them down, and keep going with the circular motion. Try to be mindful of details so you don't lose definition as you go. 

The good news is with each finer grit, you don't need to spend as much time sanding that layer. 


1000 Grit Smooth

 (after using 1000 grit sandpaper it'll start to look really smooth)


3. Keep working up the sizes, from 400 to 600, then to 800 and upwards. Don't miss out steps when you get to 2000. It'll feel really smooth, and will polish nicely at that level - but if you want a mirror finish you need to build to 3000. 


3000 Grit Smoothed 3D Benchy

(here we're at 3000 grit; you can start to see the shine before you've polished)


4. The final (and most fun) part is the actual polishing stage. Layering up those sandpaper increments doesn't feel like you've made much progress, but once you polish up, the final finish looks great. 

To really smooth out 3D prints to that mirror finish we've found, you just need any good liquid metal polish. Our favourite is the ol' trusty Brasso. 


Brasso To Polish 3D Prints


The technique to polish your prints is simple. Get a soft cotton cloth, and apply a small amount of liquid polish. Then apply to your 3D print surface in small circles until all the polish is rubbed into the surface. 

Once you've finished that, the surface should look reasonably shiny. The final stage is to buff up the surface into that mirrored, polished shine. Just get an unused area of your cloth, without any polish on, and go back over the area you've just worked on to buff it up to a shine. And that's it. 

Here's a before and after reminder on the technique: 


Before and after smoothing and polishing PLA prints


Hope you found this guide to smooth out PLA 3D prints useful. If you did, why not subscribe to our blog to receive updates on other helpful 3D printing how-tos. 

Please comment below if you have any tips to add or questions - we'll be only too happy to help. 

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