I've done plenty of mockery of people who don't know how to sand their fucking prints but I figured maybe it's time to stop mocking and start teaching. So, here is:
THE SEMI-PROFESSIONAL PROPMAKER'S GUIDE TO MAKING YOUR 3D PRINTS NOT LOOK LIKE CRAP
There is no singular fixed process involved here, but a variety of techniques that fall into a vague and often overlapping timeline.
First off, what do we mean by 'finishing' 3D prints?
What we mean is getting rid of these:
LAYER LINES. A byproduct of how 3D printers work. A 3D printer in its software slices a 3D model up into thin layers (usually about a fifth of a millimetre thick) that it then draws out with its toolhead using plastic filament, stacking the layers on top of each other until the digital model is made physical.
If you use 3D printers for the same reasons I do, chances are you're trying to make your 3D print not look 3D printed. These layer lines are a dead giveaway so they have to go.
STEP ONE: Things you can do WITH your printer.
Top of this list is simply reducing layer height
Advantages: the thinner the layers, the less pronounced the lines, the less cleanup you have to do. If you REALLY don't want to do the tedious work of cleaning up an FDM print you can print in resin with layer lines an order of magnitude thinner, though resin printing is its own can of worms.
Disadvantages: TIME. Thinner layers means more layers means longer print times. Halve your layer height, double your print time.
also, the 0.4mm extruder nozzle that comes standard on most FDM printers struggles to consistently extrude that little filament. If you plan on printing at these layer heights a lot, get a smaller nozzle.
Next thing you can do (depending on your slicer software): Ironing
Ironing is a relatively new feature that tells your printer's nozzle to run over top layers again, with a different pattern, extruding no filament. The heat of the nozzle melts the plastic down to create a smoother top layer.
It's not perfect but it can make a big difference. It's especially useful for smoothing top layers you can't get to with sandpaper. Again, adds print time and not all slicers have this feature available.
Slowing down your print speed will offer smoother prints but we're getting into diminishing returns on cleanup time saved now.
If you're printing the same sorts of things as me (props, costume pieces, and models) you're probably printing in multiple pieces that need to attach together. As a general rule of thumb: Welding is stronger than mechanical connections (screws, bolts, staples, rivets, etc.) Which is stronger than glue, which is stronger than tape. Let's go down the list.
WELDING: it is entirely possible to weld 3D printed parts together. They're not conductive though so, while I know you were excited to use it, I'm gonna have to ask you to put your MIG rig away. What you CAN do however is stick some filament in a dremel and friction-weld those suckers together.
I've never done this before so I can't speak to its efficacy but it sure is a thing you can do.
You can achieve similar results with one of those 3D pens that were popular a while ago or some filament and a shitty old soldering iron.
MECHANICAL CONNECTIONS: Because most 3D prints are hollow-ish, mechanically fastening 3D prints together takes some forethought while you're still modelling. If you want to add screws, you need to put screw holes IN the model.
Though there also exist plastic welders
these tools basically melt staples into the seams of your pieces. It can make for very strong connections, but it does little to make a clean seam.
That's where we get into:
GLUE. Just glue is totally fine. The above results are... exotic. I haven't done most of that shit.
Most common is cyanoacrylate glue, better known as superglue. Nothin wrong with it, works well and works fast. Only downside is it's brittle. Any twist or shear and it'll pop apart.
For anything more loadbearing I recommend epoxy. You can get tubes of five-minute epoxy at the dollar store. Squeeze out a bit from the tube, mix the two parts together, apply with a brush or popsicle stick to your parts and stick together within five minutes before the epoxy starts to cure (hence the name).
Industrial adhesives like E6000 also work pretty well on prints though I have not used them enough to say more.
STEP THREE (the most important step): SANDING
this is the one I know you've heard of. You gotta sand your prints. Even if you're completely fine with layer lines for some reason you still need to break the smooth plastic finish to give it any kind of paint treatment.
The general rule of thumb with sanding is to start with a low grit and work up. The higher the grit, the smaller the particles on the paper.
on 3D prints we can go as fine as 1000 grit depending on what we're doing.
While sandpaper itself is perfectly fine there are a few other sanding tools worth mentioning.
Sponges work just like regular sandpaper with the distinct advantage of being soft, squishy, and flexible. They're excellent on curved surfaces and can be folded to make a finer point and rolled up into small areas. Definitely get yourself some sanding sponges.
When humanity invented sandpaper the next step was obvious. Do what humanity does best with new inventions: put it on a stick. Hitting things with rocks? Put the rock on a stick and now you have more hit per rock. We call this: a hammer.
Sandpaper on a stick allows for more pressure and more precise control.
Metal files are able to take large amounts of material off quickly, but leave scars. Metal files are best used early on in the process, ESPECIALLY before priming because the paint will gum them up and reduce their effectiveness.
Nail files can do much of the same job on a tight budget, with two other advantages: they can be cut down to a specific shape, and if you get them full of paint, oh well! They're disposable.
Dremels, palm sanders, belt and disc sanders. I'm not saying never use them, but you REALLY gotta be careful. They can get a LOT of sanding done FAST, but sanding works on friction and friction generates heat. 3D printing plastics are VERY temperature sensitive and if you overuse a sander you WILL melt your piece. Short controlled bursts.
Sometimes sanding alone isn't enough. Sometimes your prints still have blemishes, gaps, and seams that need to be filled. I call this step four but sanding and filling are usually a back-and-forth process.
Let's start with what was my main filler for years: good old fashioned...
Sticky paint in a spray can. Any paint fills gaps. Chances are if you're doing 3D printing you already have a can or two of spray primer lying about. It's slow and inefficient as a filler, but it's cheap and was my workhorse filler for the better part of a decade. Days, if not weeks on end of back-and-forth sanding and priming and sanding and priming and sanding and priming and sanding and priming and sanding and priming.
There are also filler primers but in my experience these fill details you wanted to keep while doing fuckall for layer lines. Don't do this to yourself. There is a better way. Come. Take my hand.
Y'know what else has a plastic shell that frequently needs holes dents and scratches filled? CARS! This means there are lots of very useful items for us available in the automotive aisle of your local hardware store
BONDO: A two part mixture for filling in the big gaps.
Like most automotive goos, bondo is as toxic as your ex. Yeah that one. You know which one I'm talking about. Use outside or use gloves and a respirator. Bondo is rock-solid once it sets but doesn't sand well. If you use bondo, you gotta get it smooth BEFORE it dries or you're just gonna frustrate yourself.
but now we get to my personal favourite. This is a product I had been aware of for years, but a buddy of mine last year showed me its full potential:
While not nearly as strong as bondo, spot putty fills gaps just as well but sands BEAUTIFULLY. Still toxic, so gloves and respirator, but oh my god.
You can use it straight out of the tube to fill big holes but this is the thing that made me fall in love: THIS CAN BE THINNED. You can thin it down with acetone or isopropyl alcohol (though acetone works better) and apply it LIKE A PAINT!
and once it's dry it sands just as well as the full thickness stuff. I love spot putty.
last option I know of, out of the automotive aisle this time.
UV RESIN: a special liquid resin that cures and solidifies under ultraviolet light. You may know it as the resin used in resin 3D printers. That resin works in a pinch but there are specialized resins meant for filling gaps. It's not high on my to-try list but it might work for you who knows.
Past this point we go into painting which is another post for another time. This should be enough info to get you to that point. Now go forth make beautiful printed props and models.
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