How to sand a 3D print smooth

Sanding is the stage where people put in the most effort and get the least back, and it’s nearly always the same two reasons. They start with paper that’s too coarse, and they start before the surface is flat. Both are free to fix.

Do two things before you touch the paper

Flatten the high spots with a file, not with paper. Sandpaper is backed by something soft, usually your finger, so it follows whatever shape it’s pressed against. Put it over a support nub standing proud of a surface and it wraps over the nub and takes material off the flat around it instead. A needle file, which is a small steel file about the size of a pencil, is rigid, so it can only cut what’s sticking up, and it stops cutting by itself once the nub is level. There’s more on that in removing supports without marks.

Decide what the surface is for before you start. It changes where you stop, and sometimes whether you bother. A face that’s getting painted wants to stay slightly rough, because paint grips a scratched surface and slides off a polished one. A part staying bare wants to go much further. Sanding a face to a shine and then spraying primer over it is work you threw away.

Why PLA has to be sanded wet

PLA starts to soften under load at 55°C. That’s not far above a warm day, and rubbing paper over plastic puts a surprising amount of heat into the top skin of it. The surface softens, the dust stops being dust and turns into tiny soft balls, the balls clog the paper, and from then on you’re dragging hard lumps across the surface you’re trying to smooth. Most people read that as cheap paper. It’s the plastic melting.

Water fixes all of it. Dip the part, dip the paper, keep both wet, and the material comes off as a grey slurry that rinses straight off. The paper lasts several times longer, there’s no dust in the air, and the cut is more even. A bowl of water on the table is the whole setup, and a drop of washing-up liquid in it helps the paper glide.

PETG puts up with dry sanding better, because it softens higher, at 68°C. Wet is still better. Cured resin can go either way, and you want it wet for the dust rather than the heat.

Which grit, and in what order?

The numbers on sandpaper are how coarse it is, and lower means coarser. Each grade exists to remove the scratches left by the one before it. That’s the whole logic, and it’s why skipping grades doesn’t save time. Go from P180 straight to P800 and the P180 scratches are still there under a polish, which looks worse than leaving it rough.

• P180 to P320. File marks, support stubs, the ridge round the bottom of a print. This grade does the shaping, most of your time goes here, and the rest is quick if you do it properly. It’s also the one grade most finishing kits don’t carry, mine included, so a sheet of P180 wet-and-dry from a hardware shop is worth picking up with everything else.
• P400 to P600. Takes the coarse scratches out. A part that’s getting painted stops here.
• P800 to P1000. Where a bare part stops. The surface stops catching the light at an angle and starts looking moulded rather than printed.
• P1200 and up. Only worth it for gloss or clear parts, and then you carry on into polishing compound rather than stopping at paper.

Turn the part 90 degrees between grits. The new scratches then run across the old ones, so you can watch the old ones disappear. When every scratch on the face runs the same way, the grade before is gone and you can move up. Without that trick you’re guessing.

One thing about the numbers themselves. A P in front means the European grading scale, a bare number usually means the American one, and above roughly 220 they stop matching, with the P version being the coarser of the two at the same number. Cheap sanding sponges, including the ones in my own kit, don’t tell you which scale they were graded to. So use the numbers as an order to work through rather than as exact sizes, and trust what you can see on the part over what’s printed on the back of the sponge.

How to actually do it

• Back the paper with something flat for flat faces. A lolly stick, an offcut of plastic, anything rigid. Paper on a fingertip rounds edges over, and a rounded edge where there should be a crisp one is what makes a part look home-made.
• Use something that bends on curves. A sponge or a flexible pad follows the shape instead of flattening the high points of it.
• Blend outwards. Work the mark, then feather into the surface around it. One face sanded hard next to untouched surface leaves a visible patch, and the patch reads worse than the scar did.
• Even pressure, and let the grit do the cutting. Pushing harder mostly puts in deeper scratches you’ll have to remove later.
• Check with a fingernail, not your eyes. A nail finds a ridge long before you can see one.
• Rinse the part between grits. One stray coarse particle carried up to the fine stage undoes a whole grade in one stroke.

And the trap everyone falls into once: a wet surface always looks better than it is. Water fills the scratches and stops them scattering light, so a part looks finished while it’s wet and rough the moment it dries. Dry it and look at it under a light at a low angle before you decide you’re done.

Why sanding won’t fix layer lines on a curve

On a flat upright wall, sanding takes layer lines off and that’s that. On a curve, a dome or a gentle slope, the lines are the surface stepping round the shape, so sanding knocks the tops off the steps without reaching the bottoms. You get a wavy face with the lines still visible, and more sanding makes the waviness worse.

What works there is filler primer, which is spray primer with enough solid in it to fill scratches. Spray a coat, let it dry, then sand it back with P400. The primer sits thickest in the dips and thinnest on the peaks, so the paper takes it off the high spots and leaves it in the low ones, and each round levels the surface instead of following it. Two or three rounds and the lines are gone.

Primer is also the best inspection tool you’ll ever use, because one flat grey coat shows every low spot and scratch you missed, which raw plastic hides. There’s more in how to paint a 3D print.

When do you stop?

• Getting painted: P400. Primer needs something to grip, and the primer and sanding rounds above do the fine work.
• Staying bare: P800 to P1000, checked dry, under a light at a low angle.
• Going glossy or clear: past P1500 and then polishing compound, and expect it to take longer than everything else on this page put together.
• A working part nobody looks at: file the nubs level and stop. Honestly. Sanding a bracket is time you could spend printing the next one.

What about the dust?

Sanding wet means there isn’t any, which is most of the argument for it. If you do sand dry, do it outside or with a mask on.

Be most careful with resin prints. Cure them fully before sanding, because uncured resin is classed as something that can give you a lasting skin allergy, and sanding slurry is a thorough way of getting it onto your hands. Cured resin also sands to a very fine dust that hangs in the air, so wet is the sensible way with it too. The full order for resin parts is in resin print post-processing.

Full disclosure: I sell a kit with four grades of sanding sponge and ten needle files in it, and its coarsest sponge is 320-400. That’s the fine end of the shaping grade above, so on a heavy support scar the files do the levelling and a sheet of P180 does the rest. Worth knowing before you buy it expecting the whole ladder. And if everything you print is getting painted, you’ll use two of the four sponges and never touch the fine ones.