Acetone has been used for some time to treat ABS parts, either by polishing, vapor smoothing, or even using it to stick parts together. Likewise Methyl ethyl ketone (MEK) has been used to the same effect with ABS. We found Tetrahydrofuran (THF) to be the best option in our assessment, but be warned that there are serious safety concerns to working with THF.
This post received a lot of attention, and a number of people have rightly brought up concerns with the safety of interacting with THF. This post describes a procedure that CAN be done, but for which there are obvious safety concerns. Please read the Safety section in detail, read the MSDS (link provided), and be responsible.
We first found and started experimenting with THF at the beginning of April. We posted our results on Google+ and have been busy ever since with what seems like everything besides getting this post written like we promised in that G+ post.
Why Would I Want to do This?
These smoothing methods smooth over the ridges formed by the layers of an FDM / FFF print, any similarly small artifacts of the printing process (such as surface finish blemishes where the part was in contact with support material, etc.), or small details. This gives parts a distinctly less "printed" look and a glossier finish. See the photos below for reference, or take a look at this google image search for ABS vapor smoothing.
You can read more about THF on its Wikipedia page. Of particular note is this little gem: "THF is considered a relatively nontoxic solvent, with the median lethal dose (LD50) comparable to that for acetone." To the best of our knowledge this is the least-toxic solvent for PLA, however, you should take extreme care in handling, as the long-term effects of exposure can be very dangerous.
Whenever you have a need to handle THF, make sure to wear gloves, as it will quickly dehydrate skin, potentially dangerously. Make sure you don't use latex, as it will eat through it, use nitrile or neoprene gloves, as the Wikipedia article also mentions.
Update: As with acetone and many other solvents, the fumes can be strong and dangerous, make sure to do this in a very well ventilated area or outdoors. THF is flammable, so be careful not to expose it or its fumes to open flame or other sources of ignition. The same goes for acetone or MEK, etc.
Additional Update: As some have pointed out, you might note from the MSDS (linked below) that THF is a carcinogen and mammalian mutagen. Repeated or prolonged exposure can cause organ damage. This is a serious industrial chemical and should be treated as such. Please treat this post as informational and do not attempt working with THF unless you are positive you can do so in a safe manner.
Do your homework, read the MSDS (PDF Direct Link), and act responsibly. Only you are responsible for your safety.
The only alternative we are aware of for processing PLA is dichloromethane, which has many of the same safety concerns but harsher acute effects, and appears to remain in the plastic longer, increasing risk of exposure.
Please also note the safety consideration mentioned under the vapor treatment section.
The procedure for vapor treating PLA with THF is essentially the same as for treating ABS with acetone. The PLA object to be smoothed is placed on a non-soluble support of some kind (an aluminum foil raft, a wire rack, or some other sort of scrap, etc) inside of a closed vessel with a non-air-tight lid. Heat is then applied to evaporate the THF so it can interact with the object's surface.
For more information about the procedure and arrangement, take a look at this post about vapor treating ABS.
As our example, here is a shark tooth printed in our Brown PLA on our Makerbot Replicator2, then smoothed in a chamber and allowed to dry.
Note the rough artifacts at the top. When this part was treated, it was resting on those points, creating those rough spots when removed. When treating parts, make sure and be deliberate about the surfaces the part is resting on. Also note that the smaller your container is, the more difficult a time you may have getting an even distribution of gas, so you may get uneven smoothing.
Now, the extra safety warning you were pre-warned about. Note this passage from the Wikipedia article on THF:
The greatest danger posed by THF follows from its tendency to form highly-explosive peroxides on storage in air. To minimize this problem, commercial samples of THF are often inhibited with BHT. THF should not be distilled to dryness, because the explosive peroxides concentrate in the residue.
What does that mean to you? Well, it means you should probably be doing this outdoors, and should probably not let it dry out. Wash out your vessel between uses. In general, just make an effort to be really safe and take all reasonable precautions to avoid burning your house/business down or blow anything up unintentionally.
THF Hand Polishing
Vapor treatment not your style? It also works to hand-polish using THF on a cloth.
To do this, use a lint-free white (or otherwise non-dyed) cloth that you're willing to devote specifically to this purpose. Once you've used a cloth for this it will have PLA bound in it, making it less suitable for many other tasks.
Once you've located an appropriate cloth, don some nitrile or neoprene gloves, head to a well-ventilated area, and wet the cloth with the THF. Simply polish the part with the wet bit of cloth. The direction of your strokes may have an influence on the finish you'll create depending on part geometry and the relative smoothness or roughness of the cloth you're using. Circular motion is generally a safe bet in the absence of another preference.
Allow the part to dry for a bit (letting the excess THF evaporate out of the surface) and you're good to go.
Here's an example of a polished shark tooth printed in our ivory PLA on a Makerbot Replicator2.
Note that these photos are in focus. They were taken with a macro lens and have a relatively narrow depth of field, so only the main focal point is focused, but it is in focus even though the edges of the part look blurred.
The only portion of this part that was polished was the center, where the highlight is. The photo doesn't do it justice, it has a very nice surface finish. Compare the finish where it was polished to the left side where the ridges from the layers are visible.
Obviously, the smoother your part is, and the higher the resolution (so long as your printer is well-calibrated) the less time you'll have to spend polishing to get a nice, smooth finish.
You might also note a slight bit of discoloration in those photos. This was actually slightly more pronounced in person. This occurred because the cloth we were using had a dyed pattern on it, which presumably went into solution and rubbed into the plastic.
Where Can I Buy THF?
This is the THF we acquired (and obviously, what we used):
We bought ours on Amazon. Make sure to buy one with an inhibitor to help keep you from blowing up your shop. Seriously.
There are also regulations on where THF can be shipped. It may be the case that you can only acquire THF as a business purchase. Make sure to investigate any laws or regulations regarding your ordering THF and communicate with the seller to ensure your order will go smoothly.
Update: Other Finishing Methods
It is also worthwhile to note that PLA can be sanded fairly easily. For some applications sanding may be sufficient to get the surface finish you'd like, particularly if you just want to remove artifacts from support material or any other surface blemishes. Sanding would also work as an excellent pre-process to treatment with THF. Just give your part a quick sand, particularly in troublesome areas or particularly large blemishes, then treat as described above. It should save you time and solvent and will allow you better smoothing with less destruction of fine geometry.
PLA can also be glued with a variety of adhesives (epoxies and super glue have both worked well for us).
Good luck, stay safe, happy printing!