Post-processing and Finishing

Plan the finish before you print

A finished surface is easier when the print is designed for it. Print the visible faces on the bed or vertical, use a finer layer height where the surface matters, put the seam and the supports where they will not show, and leave 0.1 to 0.2 mm of extra material on surfaces that will be sanded, filled or machined. Most finishing adds thickness (primer and paint each add about 0.05 mm per coat) or removes it (sanding), so allow for that on parts that must fit together.

Removing supports and cleaning up

• Let the part cool on the bed first; many parts release by themselves, and a PEI spring sheet flexes the part off.
• Pull off supports with pliers and flush cutters, working from the outside in; tree supports peel away in sections, while grid supports break off in sheets. A hot plastic is softer: a heat gun warms a stubborn support.
• Clean up the contact surface with a sharp knife or a deburring tool (a hobby knife with a fresh blade), always cutting away from your hand, and wear eye protection for flying chips.
• Remove strings and blobs with a lighter flick (carefully) or a heat gun, or trim with side cutters.
• Brims and rafts peel off; trim the edge with a knife and sand flush.
• Soluble supports (PVA, HIPS): soak in water or limonene until they dissolve.

Sanding and filling

Sanding removes the layer lines. The ridges on a gently sloping or curved surface are about the layer height times the sine of the slope angle deep (0.20 × sin 30° = 0.10 mm for a 0.2 mm layer on a wall leaning 30° from vertical), so a coarse paper must remove roughly that much material before a finer paper polishes the scratches.

Grit	Particle size (approx.)	Use
P80	≈ 190 µm	Shaping and removing layer lines
P120	≈ 125 µm	Shaping and removing layer lines
P220	≈ 68 µm	Smoothing after filler or primer
P400	≈ 35 µm	Before paint, wet sanding
P800	≈ 21 µm	Final polish, wet sanding

• Work through the grits from coarse to fine without skipping more than one step (for example 120, 220, 400, 800), and wipe the dust between grits.
• Wet sanding (water with a drop of soap) keeps the plastic cool and the paper clear: dry sanding PLA and PETG heats the plastic and loads the paper, smearing it.
• Fill gaps and low spots with a filler primer, a thin coat of epoxy resin, polyester or automotive putty, or a plastic-specific filler, then sand again. Several thin coats beat one thick one.
• Dust: sanding makes plastic dust; wear a mask (P2 or FFP2) and eye protection and work outdoors or with extraction. Wet sanding avoids most of the dust.

Priming and painting

• Clean the part (wash with soap and water, then isopropyl alcohol) and let it dry.
• Prime with a spray filler primer (two to three thin coats, about 0.05 mm each: 3 coats add about 0.15 mm to every surface), sand with 400 to 800 grit, then prime again until the lines are gone.
• Paint with acrylic or enamel spray in thin coats, 20 to 30 cm from the surface, with 10 to 15 minutes between coats, then a clear coat. Test on a sample first: some paints attack some plastics.
• Brush and airbrush acrylics suit models; a matt finish hides remaining lines.
• Dip and cold-water transfer and vinyl wraps give patterns and textures.
• Allow for the paint thickness on mating surfaces: a part that fits with a 0.2 mm gap per side may bind after three coats of primer (about 0.15 mm) and two of paint.
• Work in a ventilated place and follow the spray can’s safety label.

Smoothing: acetone, heat and coatings

Method	Works on	Notes
Acetone vapor	ABS (and some ASA)	The part is held in acetone vapor (in a closed container, never touching the liquid), which softens the surface so that the lines flow together. Gives a glossy, sealed surface; it also rounds sharp edges and details. Acetone is flammable and the vapors harmful: use in a ventilated place, away from flames, sparks and heaters
Ethyl acetate, THF or special solvents	PLA, PETG (some)	Less effective and more hazardous; test carefully; many makers avoid these. Use the solvent’s safety data sheet
Heat gun (quick pass)	PLA, PETG, ABS	Smooths strings and fuzzy surfaces, but it also distorts thin walls: use light, quick passes
Epoxy coating (XTC-3D and similar)	Any plastic	A thin brushed coat (about 0.2 mm) fills the lines and cures to a glossy hard finish; adds the coat thickness to every surface
Sanding plus clear coat	Any plastic	The most controllable method: no distortion

Annealing for strength and heat resistance

Annealing heats the printed part above its glass transition temperature for some time, letting the polymer chains relax and (for crystallisable plastics such as PLA, PETG and nylon) crystallize, which raises the heat resistance and sometimes the strength and layer bond. For PLA the part is held at about 80 to 110 °C for 20 to 60 minutes (and embedded in sand or salt to keep its shape), then cooled slowly in the oven. Only special PLA grades designed for annealing keep their dimensions well; ordinary PLA and PETG shrink and distort.

• Do not anneal parts with tight fits, thin walls or an internal air gap unless tested; use sand, salt or a mold as a support.
• Nylon and PETG can be annealed in an oven too, but follow the maker’s instructions.
• A safer alternative for heat resistance is to print in ABS, ASA, nylon or polycarbonate.

Joining printed parts

Method	Best for	Notes
Cyanoacrylate (superglue)	PLA, PETG and ABS small parts	Fast; brittle bond; use gel for gaps and an activator for speed; ensure good fit
Epoxy (two part)	All plastics, dissimilar materials, gap filling	Strong; slow; roughen the surfaces first
Solvent welding	ABS and ASA (acetone), PLA and PETG with special solvents	Dissolves the surfaces so that they fuse into one: the strongest joint, but needs a close fit and a ventilated place
Plastic welding	Large parts	A soldering iron or a plastic-welding tool melts the joint and a filament adds the filler
Screws and inserts	Parts that must come apart	See Designing for 3D Printing
Press fits and snap fits	Tool-free assembly	Design them with clearance tests; see Designing for 3D Printing

Machining and drilling printed parts

• Drilling and reaming give accurate holes: drill slowly with a sharp bit and light pressure, at low speed (to avoid melting the plastic), and back out often to clear chips. A reamer finishes the hole to size.
• Tapping works in solid walls of at least 2 to 3 mm: use a tap with lubricant, and keep the thread engagement short and the load light; or use inserts.
• Milling and turning a printed part is possible when it has solid walls: leave 0.3 to 0.5 mm extra for the cut, use sharp tools, light cuts and no coolant that attacks the plastic.
• Fixture: support thin walls so they do not flex or chip; a printed part can be clamped lightly in soft jaws.
• See CNC Machining for plastics machining basics.

Finishing resin and powder parts

• Resin parts: remove supports after the wash and before or after the cure (see Resin and Powder Processes); sand wet from 400 to 2000 grit, prime, paint, or polish with a plastic polish. Cured resin is hard and brittle and sanding makes a harmful dust: wear a mask and work wet.
• Clear resin can be made transparent by wet sanding to a high grit and coating with a clear coat or dipping in clear resin and curing it.
• SLS and MJF parts: media-blasted, tumbled, dyed (black is the most common) or vapor smoothed; they take paint with a primer.
• Metal parts need support removal, heat treatment, machining of critical faces and surface finishing.

Related

• 3D Printing and Additive Manufacturing: the overview of this guide.
• Designing for 3D Printing: design parts so that finishing is easy.
• Filament Materials: which plastics sand, glue, smooth and anneal.
• Resin and Powder Processes: washing and curing resin parts.
• Workshop Safety: solvents, dust and eye protection.

FAQ

Can I smooth PLA?

Sanding, filler primer and paint are the practical routes. PLA does not dissolve well in common household solvents; ethyl acetate and similar solvents work in some cases but are hazardous, so most makers avoid them.

What glue is best for 3D-printed parts?

Superglue (cyanoacrylate) for small PLA and PETG joints, epoxy for strength and gap filling, and solvent welding (acetone) for ABS and ASA, which gives the strongest joint.

Is acetone smoothing safe?

Acetone is highly flammable and its vapors are harmful: use only a small amount in a closed container, outside or in a well-ventilated place, and keep it away from heat, flame and electrical sparks. Wear gloves and eye protection.

Do I need to sand before painting?

Yes. Layer lines show through paint, so sand and prime until the surface is smooth. Filler primer does most of the work, with two or three sanding rounds.

References

• Filament makers’ guides to post-processing (Prusa Knowledge Base, Polymaker, Formlabs finishing guides).
• ISO 6344 and manufacturers’ data on coated abrasive grit sizes (FEPA P-grade); epoxy and adhesive makers’ data sheets for shear strengths.
• Safety data sheets for acetone, isopropyl alcohol, epoxy resins and any solvent used.

Last reviewed: October 2026. Adhesive strengths and annealing results are typical: test on a sample.