FDM Post-Processing: support removal, sanding, painting and gluing — and how to price it

An FDM part rarely goes to the customer straight off the bed. Layer lines are visible to the eye, support contact points leave rough spots, and if the customer wants a solid color or a smooth surface, post-processing is unavoidable. This is a different set of mechanics from resin printing — that's about washing and UV curing, this is about the mechanical and chemical finishing of a layered plastic surface. If you're working with resin, you probably want the separate article on post-processing resin prints — the steps there are different.

Support removal and rough prep

Supports come off with flush cutters or pliers right after printing, while the plastic is still slightly warm and more pliable — they snap off cleaner that way. Contact points where supports touched the part almost always leave burrs and rough patches, which is the first thing to clean up before anything else, usually with a hobby knife or a small file.

Sanding

Visible layer lines are removed with sandpaper, working up through grits — for example, 200 → 400 → 800 → 1200. Don't skip several grit steps at once: coarse paper leaves scratches that fine paper won't remove in a reasonable amount of time.

A few practical notes:

• Wet sanding (sandpaper plus a little water) cuts down plastic dust in the air significantly and makes the sandpaper last longer.
• Filler primer before the final sanding pass helps — it partially fills the layer lines, and after it dries, sanding takes less effort and gives a smoother result.

Acetone vapor smoothing — ABS only

A separate technique for ABS is smoothing the surface with acetone vapor: the part goes into a sealed container with acetone (for example, an acetone-soaked cloth at the bottom, or a proper vapor chamber), the vapor slightly melts the outer layer of plastic, and the layer lines visibly disappear after a few minutes of exposure. Timing matters — leave it too long and the part loses fine surface detail and can start to lose its shape.

This method doesn't work on PLA or PETG — acetone barely dissolves them. For those materials, mechanical sanding, or a different solvent matched to the specific plastic, is the only option.

Acetone is highly flammable and its vapor is toxic with prolonged exposure — work in a ventilated space, away from open flames or spark sources, wearing solvent-resistant gloves.

Priming and painting

Even after sanding, layer lines are often still faintly visible at certain angles of light — a filler primer under the paint smooths them out completely. Painting is usually done with plastic-compatible acrylic spray paint, applied in several thin coats — that gives fewer drips and a more even finish than one heavy coat.

Gluing parts together

For most FDM plastics, cyanoacrylate (super glue) is the practical choice: it sets fast and is strong enough for decorative and lightly loaded joints. For more heavily loaded mechanical joints, two-part epoxy is a better fit — it's stronger and gives you time to adjust part alignment before it cures.

ABS specifically also supports solvent welding: acetone partially dissolves the plastic surface at the joint, and once the solvent evaporates, the parts are effectively fused together — the resulting joint is often stronger than a glued one. Dichloroethane was historically used for the same purpose, but it's toxic and carcinogenic — acetone gives a comparable result on ABS with much lower risk, and there's no good reason to reach for dichloroethane instead. Whichever solvent you use, ventilation and solvent-resistant gloves are non-negotiable.

How to price it in

Post-processing is mostly time, not consumables: sandpaper, primer and paint cost pennies per part, but 15-40 minutes of hands-on work removing supports, sanding and painting a single piece is a real line item in the cost — one that's easy to miss if you're only pricing by plastic weight and print time.

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The logic is the same as for resin post-processing: if a customer orders a part meant for painting, price the time for it as its own line item instead of folding it "roughly" into the order total — otherwise orders that need post-processing end up being the least profitable in the lineup, even though they look the most premium.

Bottom line

FDM post-processing is a sequence: support removal → sanding (with acetone vapor as an option for ABS) → primer → paint → gluing if needed. Every step takes real hands-on time, and that time — not the consumables — is what actually drives the cost of this stage, so price it in explicitly.