The method first starts with some good old fashioned detail sanding working up through the grits, from very rough, to a very fine wet sanding. Then we move into two different grits of polishing compounds, and finally finish everything off at the buffing wheel using two very fine polishing abrasives, and finally, a carnuba wax polish to buff everything to a bright shine.
When done correctly, well polished resin casts and parts look absolutely amazing, and are works of art, ready for display, jewelry and enjoyment in and of themselves.
Before I get too far into things, let me just tell you a little bit about the part that we'll be polishing. A few of us at Instructables have been working with a lab at UC Davis to do some test prints of 3D models that they've been developing to better understand how and why race horses break their bones. This Instructable focuses only on how to polish these kinds of models. There will be a much larger Instructable about the entire research project coming along soon.
The particular model featured in the photos in this Instructable is a horse's leg muscle that's been scanned and modeled to show how tendon attaches to the end of a horses muscle. Tendons are modeled in white, while muscle tissue is represented in clear. While casting a resin part like this would be extremely difficult, and realistically speaking, virtually impossible, our Object 3D printers can produce a resin print like this a matter of hours.
I've been excited to work with the researchers at UC Davis because I find this kind of 3D printing really compelling when it comes to showing off the kind of work that we're capable of producing with this new technology. There's obviously a whole lot to be explored with 3D printing, but representing complex biological models to advance medical research is one of the more compelling applications.
In any case, if you've ever wondered how to make resin shiny, read on.
Step 1: Clean 3D Print or Cast Part
Depending on how the part was cast or printed, it may or may not have a rough surface. For the purposes of this Instructable, we are going to assume that the part has a rough surface, since the steps in polishing something that is rough are the most comprehensive and encompass the steps for simply polishing something that is already smooth.
If your surface is already smooth, it's possible to skip some of the first few steps that are a part of this Instructable.
Step 2: Tools and Materials
- sand paper 120, 180, 220, 400, 600, 800, 1000, 1500, 2000 grit
- sanding block
- scratch remover compound
- polishing compound
- lint free, scratch free polishing cloth
- mothers polishing ball
- buffing wheel
- buffing compounds
- wax product
Step 3: Sand Paper Grits - From 120 to 2000
The general work-flow here is to sand the model, then sand some more to be sure you've covered all the areas. Watch out for concave spots, as the small divots and pits in a model are a lot more difficult to sand than convex areas. Once you are sure that you've covered every bit of the resin part, only then move up to the next finer grit paper. Starting around 400 grit, you can switch to a wet sanding method, as those papers will be specially made to work best with water. Until then, it's all just dry sanding.
Make sure to wear a mask while doing this dry sanding since it creates a lot of resin dust, and that stuff is definitely not great for the respiratory system.
Step 4: Hand Sanding
We used the sanding block only for the one flat side on this model, the rest of the time we just used our hands as a backer. We experimented a bit using flexible foam sanding pads, and sponge-like backers on the sandpaper, but all in all, our hands proved to be a pretty effective and flexible support for the paper to get inside all the detailed contours and crevices.
Step 5: Power Sanding
Once we got to the finer wet sanding steps, we quickly discovered that we had missed many spots. Since we had to virtually start over at the beginning and were already pretty tired from hand sanding, we used a detail sander to speed up the process.
My shop mate luckily owns a Fein brand detail sander. While a dremmel could also work, the quality of the Fein sander is very good. I'm sure there are other substitutes, or larger sanders that one would want to use if you were working with a larger piece of resin.
It's easy to make custom sanding pads of the finer grit sanding papers for the detail sander. Simply take a spent Fein brand pad, spray it down with 3M adhesive, and slap a piece of fine sandpaper on it that's been cut down to the same size. Presto - cheap and reusable fine grit sanding pads.
Step 6: Wet Sand by Hand
Use copius amounts of water as it helps control the very fine dust this step produces and keeps the sand paper from filling up with material.
As I said before, make sure that you are as sure as you can be before moving up to a higher grade sandpaper. Failure to thoroughly sand an area will show right through as things clean up and become more polished.
Step 7: Check for Scratches
If the resin passes the scratch test, keep on sanding all the way up to 2000. By 2000 it should start to look pretty clear, but not polished. That comes in the next step.
Step 8: Rubbing Compound
Follow the instructions - apply a small amount to a clean polishing cloth and rub it vigorously onto the surface of the resin. Let it sit for a second and then buff it off with another part of the cloth.
This polishing compound is a finer grit than the sandpaper, but not quite as fine as the buffing compounds that are used on the mechanical buffing wheel.
Step 9: Polishing Compound
This creamy white polishing compound really starts to bring out the initial polished surface in the resin...and in the next steps it just gets better and better.
Step 10: Mechanical Polishing With Mothers Ball
Lock the drill trigger on, load the ball with polishing compound and polish all the surfaces of the resin piece. Hold on to the resin piece as the drill spins. It doesn't have much friction, but if it caught en edge on the resin piece, it's possible it might try to come away from your hands.
Where an apron as this step tends to flick polishing compound around and about.
Since this photo was taken I've begun polishing these models on the drill press, as it spins the ball with a bit more stability and speed control.
Step 11: Buffing Wheel - Tripoli
Loading the wheel with compound means holding the bar to the wheel for a few seconds. Once some red material is deposited on the wheel, it's good to go. If you are buffing a large area, you may need to reapply compound to the wheel.
Hold the resin piece tightly in your hands and engage it into the wheel.
Always have the wheel spin down and away from you, and always buff on the bottom of the wheel in case something catches an edge - otherwise it will get launched right at you. If you buff down and away, it will at least get launched away from the operator.
Always hold on to the part you are buffing with two hands, and try not engage any hard edge against the wheel. Better to buff the surface of the resin part parallel to the face of the buffing wheel, and never perpendicular to it as it has a tendency to "grab" the part.
Keep the resin moving across the buffing wheel, never staying in the same place too long. The process builds up significant heat and we want to keep things cool by keeping everything on the move.
Step 12: Buffing Wheel - White Diamond
It's possible to buff the entire model - even concave areas by using different parts of the wheel, and even digging into the corner of the wheel if needed for smaller, detail areas.
Step 13: Finish With Carnuba Wax
Swap wheels one last time to the all flannel wheel and load it with just a little bit of carnuba wax. The more wax you load on the worse the final buffing will be since too much wax will build up upon the surface and ruin your finish! Just touch the carnuba wax bar to the wheel and load it up for a second or two. You can always reapply more later.
* Please note, in some of the past photos on this project not all of the surfaces on the resin part are polished. We first tested on the flat surface, thinking it would be easiest, and then when the process worked great - we went back and repeated everything on all the surfaces - which were really just as easy to polish as the flat surface. We just happened to have pictures from the first go around, and not the second. Please don't get confused or worried if it looks in the photos like the non-linear surfaces of the horse muscle aren't as shiny as the linear end just yet.
After the wax is buffed on and off, your part is through being polished and should be gleaming. The resin part I was polishing in this project was hard to photograph it was so shiny any bright! I'm sure that there are many ways to alter this process, and some might even find steps which they can omit, as I literally attempted to preform as many polishing steps as possible to learn about how this process works.
Finally, as I said before, if your resin cast starts off with a relatively smooth exterior, you may be able to skip some of the initial mechanical sanding.
Please post additional tips in the comments below - polishing 3D prints is new to a lot of us, and learning what works, and what works best will help us all in the long run.
Step 14: How to Polish Small, Highly Detailed Parts Hands Free (hopefully)
I hope to be able to achieve the same results over several hours or days of parts vibrating in different grit medias that could run unattended, except for the changing of the media as the part moves up the grits.
I'm also going to test spraying a clear high gloss resin, as well as flame polishing any surface too detailed, or too small to sand in order to achieve a glossy finish.
I'll be sure to link to my findings once my tests are done and I write everything up.