Key takeaways
- The order can't change: remove → cut → sand → finish. Painting over uncut support stubs means painting twice, and sanding after painting means losing the paint.
- Skipping grits doesn't save time. A 180-grit scratch only disappears when you go through 320; jumping straight to 800 just polishes that scratch.
- Acetone vapour only works on ABS and ASA. PLA doesn't dissolve in acetone, and on PC solvents cause stress cracking rather than smoothing.
- Don't sand mating surfaces. Sanding loses the tolerance, and tolerances are fixed in the slicer, not with sandpaper.
A part straight off the build plate isn't finished. Between that moment and when it's photographed or handed to a customer there are four jobs — and in the wrong order, each one undoes the one before. This article follows that order, for FDM. Resin parts have their own process, in washing and curing resin prints.
Four steps, and why the order can't change
- 01
Remove the part once the bed has cooled
A magnetic bed with a spring-steel sheet almost always releases the part by itself when cool: plastic and steel shrink differently, and that difference peels the first layer for you. Prying a PLA part off a 60 °C bed is the fastest way to dent the first layer and bend a thin part. If you must pry, flex the sheet; don't push a blade between the part and the bed.
- 02
Remove supports and cut the stubs
By hand and needle-nose pliers first, craft knife second. The goal of this step isn't a nice surface, just no lump of plastic standing proud of the part. What's left is the sandpaper's job.
- 03
Sand through the grits
Start with a grit coarse enough to bring down the highest point, then work up. Each grit has one job: erase the scratches of the one before. This is the most time-consuming step and the only one that can't be shortened.
- 04
Only then, the finish
Primer, colour, clear coat, or solvent vapour for ABS. Every coating reveals the defects beneath it rather than hiding them — grey primer exists precisely so you can see the marks that coloured plastic is hiding.
Removing supports without leaving scars
Most pits on an FDM part aren't from the printer but from the first five seconds of support removal. Support bonds to the part at the interface, and if you yank perpendicular to the layers, what comes away is a piece of the part, not the support.
- Pull along the layers, don't yank in Z. Snap sideways, peel progressively from one edge, don't rip straight up.
- ABS and ASA come away more easily while still warm. PLA is the opposite: it softens early, and a warm PLA part deforms under the very force you use to remove supports.
- A fresh craft blade, cutting parallel to the surface. A dull blade doesn't cut, it tears, and the tear is deeper than the original stub.
- Don't try to scrape it clean with the knife. The last bit is far cheaper to deal with using 240 grit than by digging another hole to get it out.
- If support stubs bond too hard, the problem is in the slicer. Interface gap and number of interface layers decide whether support comes off easily — the support section of the handbook goes into those numbers.
Sanding: grits, and why to sand wet
Layer lines are parallel ridges. Sanding doesn't erase layer lines — it brings the peaks down to the level of the valleys, which means you're cutting into the outer wall. That's why there's a limit: over-sanding a part with only two walls breaks through into the infill, and there's no saving it.
| Grit | Its job | Stop when |
|---|---|---|
| 180–240 | Bring down support stubs, elephant's foot and high ridges | The surface feels even, not yet pretty |
| 320–400 | Erase the previous grit's scratches and most layer lines | Raking light shows no horizontal lines |
| 600–800 | A surface ready for primer | If you'll paint: stop here, don't go finer |
| 1000–2000, wet | A glossy surface, or preparing to polish | Only when the part won't be painted |
Wet sanding — dipping the paper in water with a drop of soap — does two things at once: it keeps dust in the water, and it stops the paper clogging with the plastic it just removed. With PLA there's a third reason: friction heats the surface, and warm PLA balls up and sticks to the paper instead of turning to dust.
Solvent smoothing: ABS and ASA only
ABS and ASA dissolve in acetone. Put the part in a chamber with saturated acetone vapour and the outermost few tens of microns flow and level themselves by surface tension — the layer lines disappear rather than being cut away. Among common FDM materials it's the only way to a truly glossy surface without sanding.
- PLA doesn't dissolve in acetone. However long you soak it, it won't smooth. Recipes using ethyl acetate or THF for PLA do work, but both are far more hazardous than acetone and have no place in a small workshop.
- Nor does PETG. PETG resists solvents well, which is one of its strengths.
- On PC it's worse than useless. Solvents cause stress cracking in PC: the part looks intact, then crazes along its stress lines a few days later.
- The price of a glossy surface is sharp edges. Acetone vapour rounds every crisp edge, loses small text and shifts dimensions. Don't use it on mating parts.
Which materials take which finishes
| Material | Sanding | Solvent | Painting |
|---|---|---|---|
| PLA | Easy, best wet | No | Good, over a primer |
| PETG | Hard — gummy, clogs paper | No | Needs thorough keying and an adhesion primer |
| ABS / ASA | Easy | Acetone, effective | Good |
| TPU | Very hard, paper skates over it | No | Only with flexible paint |
| PA, PA-CF | Hard, makes fibre dust | No | Poor adhesion, needs a specialist primer |
| PC | Moderate | No — causes stress cracking | Good |
| Resin | Easy, wet-sand after curing | Not needed | Very good |
Two rows deserve more. PA and PA-CF reabsorb moisture while you sand and soak them — a wet-sanded nylon part needs drying again before painting, or moisture escaping under the paint will blister it. The drying routine is in how to dry filament. TPU can hardly be finished at all: a rigid paint film on a flexible part cracks at the first bend, so with TPU the colour has to be the plastic's own — a very practical reason to choose the colour when you buy the spool.
The cheapest post-processing is the kind you don't have to do
Three quarters of the time spent with sandpaper is decided earlier, in the slicer. Before slicing a part whose surface is the product, check four things:
- Orientation. The most important face should be one with no support and not the top of a curved shape — curved tops show the staircase effect most.
- Seam position. A seam gathered on a hidden edge is one mark; a random seam is two hundred small marks across a curved surface.
- Elephant's foot on the first layer. A bulging bottom edge comes from an over-squashed first layer; fixing it with first-layer compensation takes five seconds, fixing it with sandpaper takes ten minutes per part.
- Wall count. If you plan to sand, use at least three walls. Sanding a two-wall part means sanding into the infill.
All four are in the slicer settings handbook, with their names in Bambu Studio, PrusaSlicer and Orca.
The best surface is the one you never have to touch. It's decided when you set the orientation, not when you pick up the sandpaper.
Where to stop, by what the part is for
Finish level by the part's job
- Hidden functional part
- Remove support, cut stubs. No sanding.
- Hand-held fixture
- 240 grit on edges and grip areas. Skip the rest.
- Form-review model
- Sand to 400, one coat of grey primer. No colour needed.
- Product for a customer
- Sand to 800, prime, colour, protective coat.
- Mating surfaces
- Don't sand. Fix the clearance in the slicer.
The last row is the most often ignored. An 8 mm pin sanded a few turns to "fit better" is a pin with no defined dimension, and on the next print you'll be sanding again by the same feel. The clearance between parts that fit together is a number set in the drawing or the slicer, not a manual operation.
After this step there are three branches: painting and coating, gluing and threaded inserts for assembly, or annealing if what you need is mechanical properties rather than surface.
Frequently asked
Does sanding weaken a part?
Yes, by exactly the amount of material you remove. With three or more walls that's negligible. With two walls, heavy sanding in one spot can thin it to a single wall, and that's where the part will crack. If a part has to be both strong and good-looking, add walls before printing rather than sanding more carefully.
Can a vibratory tumbler be used for 3D prints?
Yes, and it suits batches of small PLA or cured resin parts very well. It rounds edges evenly and removes support marks without hand-sanding each one. But it also rounds the sharp edges and embossed text you want to keep, so don't put in parts with fine detail or mating faces.
Why does sanded PETG turn cloudy and streaky?
PETG is soft and tough, so sandpaper drags it into strands instead of cutting it to dust. Sand wet, change the paper sooner than you think, and use less pressure. Know in advance, too: clear PETG won't go clear again after sanding unless you apply a gloss coat — the cloudiness is thousands of tiny scratches, and only a coating fills them.
Should I use filler instead of sanding?
For deep gaps and large support marks, yes: modelling putty or a filler primer fills much faster than sanding the whole surface down to the bottom of the gap. But filler only fills, it doesn't level — you still sand once it's dry. How to combine the two is in how to paint 3D prints.
How soon after printing can I paint?
PLA, PETG and ABS can be painted as soon as they're dust-free. Two exceptions: a part just smoothed with acetone vapour needs at least a few days to off-gas, because solvent left in the surface wrinkles the paint; and PA or PA-CF parts should be dried before painting, because moisture escaping later blisters the paint.