3D printing: Surface finishing – Workshop guide
Do you know this feeling? The part is functionally great, but the layer lines are annoying. Matte surfaces, small blobs at the seams, support scars – you can see everything in the light. We regularly reach this exact point with customer parts. And yes: it also happened to us in the beginning that we sanded too roughly, sprayed the filler too thickly, or applied the paint too quickly. Today, we work with a clear sequence that reliably delivers a clean finish in practice.
Why is the effort worth it? Smooth surfaces increase perceived quality – whether for prototypes at SMEs, presentation models for schools, or props for hobby makers. And: Properly prepared paints, clear coats, and coatings last longer. The following steps come from our workshop in Switzerland and are based on common best practices from the community.
Quelle: Own recording / 33d.ch
The principle: From coarse to fine – in thin layers
We work in cycles: Sanding → check → Filler/Putty → Sanding → Check → Paint → Clear coat. Apply thinly, don't skip anything. International guides recommend exactly this procedure with progressive grits and filler as a "leveling layer" – this aligns with our experience. (All3DP, MatterHackers, Prusa Blog, Fictiv)
Materials & Tools (Checklist)
- Sandpaper: P120 → P220 → P320/400 → P600/800 (optional P1000–P2000 for polishing)
- Sanding block/fine sanding sponge, water for wet sanding, lint-free cloths
- Fine putty/glazing putty (1K) or automotive putty (2K) for larger depressions
- Filler primer ("High-Build"), adhesion promoter for critical plastics
- Acrylic spray paint or airbrush, clear coat (matte/satin/gloss)
- Holders/skewers for painting, disposable cups, stirring sticks
Step 1: Prepare the raw part
Carefully remove supports (side cutters, scalpel), briefly remove strings with hot air. Then break down problem areas with P120–P150. We always use a block on surfaces – fingers tend to sand hollows. For PETG, reduce sanding speed: the material tends to smear with friction. Wet sanding helps.
Quelle: 33d.ch
Let's start with P120–P150. Sand across the layer direction so that ridges are not "milled" in. For large surfaces, be sure to use a block.
Typical starting errors (and how we avoid them)
- Too much pressure: generates heat, PLA/PETG smears → gentle pressure, block, water.
- Grits skipped: the coarse scratches remain → only work each grit until all scratches from the previous one are gone.
- Filler too early: only when the surface is uniformly matte. Otherwise, you won't save anything.
Step 2: Apply putty & filler correctly
2.1 Fine putty locally
Fill small holes/gaps thinly, leave slightly raised. After hardening, sand flat with P320–P400. Several thin layers are faster than one thick one.
2.2 Filler primer as a leveling layer
Filler acts like "sprayable putty." This works great for PLA; for PETG, we use a suitable adhesion promoter. Spray from approx. 20–30 cm, thinly and crosswise. After drying, smooth wet with P400–P600. Two to three cycles are often enough until layer lines are practically gone. (MatterHackers, All3DP)
Quelle: 33d.ch
Spray thinly, let dry, sand wet. Mark problem areas and rework them specifically – saves time.
Step 3: Paint application – adhesion before appearance
3.1 Primer → Paint → Clear coat
- Primer (grey): final adhesion and inspection layer. Reveals remaining flaws.
- Paint: 2–3 thin coats instead of one thick one. For airbrushing: several fine mist coats.
- Clear coat: matte/satin/gloss – protects and adds depth.
Proven practice in many workshops and guides – and also standard for us. (Fictiv, Prusa Blog)
3.2 Example settings (guidelines)
- Intermediate sanding: P400 → P600 (wet). For high gloss later, P1000–P2000 + polishing compound.
- Spray distances: ±25 cm, quick, overlapping passes. Avoid stop-and-go, otherwise runs.
- Drying times: Filler superficially 20–60 min, fully cured several hours (follow manufacturer's instructions).
Error pattern → Cause → Solution (Table)
| Error pattern | Probable cause | Practical solution |
|---|---|---|
| Orange peel | Sprayed too thick, too close, cold can | Spray thinner, 20–30 cm distance, warm cans (lukewarm), wet sanding with P600 |
| Bubbles/fisheyes | Grease/dust, silicone residues | Thoroughly degrease (isopropanol), work dust-free, thin coats |
| Visible sanding marks | Grits skipped | Go back to the last grit until all scratches are gone; then proceed |
| Flaking paint | Primer missing/unsuitable | Use adhesion promoter/primer, sand substrate with P600, check cleaner |
| Smeary sanding | PLA/PETG heats up | Wet sanding, reduce pressure, shorter strokes, fresh paper |
Practical example: Figure (20 cm) from PLA, 0.16 mm layer
- Preparation: Remove supports, remove strings. P150 on edges/ridges.
- Rough sanding: completely P150 → P220 until everything is uniformly matte.
- Putty: Fill seams/pores thinly, sand flat with P320.
- Filler cycle: spray thinly → dry → wet sand with P400–P600. Two cycles.
- Primer (grey): Inspection sanding with P800 wet.
- Paint: 3 thin coats (e.g., skin tone, clothing via airbrush, details with brush).
- Clear coat: 2 coats (desired gloss). Optional polish with P2000+ and compound.
Quelle: 33d.ch
After two filler cycles and clean intermediate sanding, the figure looks "as cast," details are preserved.
PLA vs. PETG – small differences that make a big impact
- PLA: is very easy to fill and paint. Be careful with heat during sanding.
- PETG: tougher, can clog sandpaper, adhesion trickier. We roughen more aggressively (P320) and often use adhesion promoter before filler/primer.
- Epoxy coating: possible for extreme smoothness/robustness; higher effort. (Fictiv)
Insights from our workshop (33d.ch)
- Marker pen check: After the first filler coat, we spray dark lines on problem areas and then sand them specifically. Saves time.
- Thin wins: Thick layers rarely save time. Three thin ones are better than one thick one – fewer runs, less sanding.
- Wet is king: From P400 onwards, we almost exclusively sand wet. The surface becomes more uniform and the paper lasts longer.
Compact workflow (as a checklist to tick off)
- clean raw part, break edges/support scars with P120–P150
- completely P220 → P320/400, until uniformly matte
- fill locally, sand flat with P320/400
- filler thinly → wet sand P400/600 → repeat if necessary
- primer grey → inspection sanding P800
- paint 2–3 thin coats
- clear coat as desired, optional polish
Recommended resources (backlinks)
- Recommended reading: All3DP: PLA glätten – Methoden & Praxis
- Recommended reading: MatterHackers: PLA schleifen, füllern, lackieren
- Recommended reading: Prusa Blog: Post-Processing Schritt für Schritt
- Recommended reading: Fictiv: Finishing-Überblick für FDM-Teile
Video tips (YouTube)
- Recommended video: Perfect 3D Print Finishes – Sanding & Painting Tutorial
- Recommended video: Ultimate Guide to Smoothing & Finishing Your 3D Prints
- Recommended video: Finish & Paint your 3D Prints – Fill, Sand, Prime, Paint
Internal link ideas (for your navigation)
- Understanding 3D printing tolerances
- Storing filament correctly (PLA, PETG, Nylon)
- Slicer settings for smooth surfaces
- Removing supports without marks
- Airbrush basics for 3D printed parts
Mini conclusion
- Sequence beats "miracle product": Sanding → Filler → Paint → Clear coat.
- Thin layers and wet sanding deliver the most consistent results.
- PLA is uncomplicated thanks to filler/primer, PETG requires more care with adhesion.
- Mark, check, rework specifically – saves about 20–30% time in finishing.
Note: Settings are examples – printers/materials differ. Test on sample parts first.