3D Printing: Surface Finishing – Workshop Guide
Do you know this feeling? The part is functionally perfect, but the layer lines are annoying. Matte surfaces, small blobs at seams, support scars – in the light, you see everything. We regularly reach exactly this point with customer parts. And yes: It also happened to us in the beginning that we sanded too roughly, sprayed on too much filler, 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 worthwhile? 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.
Source: Own capture / 33d.ch
The principle: From coarse to fine – in thin layers
We work in cycles: Sand → check → filler/putty → sand → 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 car 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 raw part
Remove supports cleanly (side cutters, scalpel), briefly remove strings with hot air. Then break down problem areas with P120–P150. For surfaces, we always use a block – fingers tend to sand depressions. For PETG, reduce print speed: the material tends to smudge with friction. Wet sanding helps.
Source: 33d.ch
Let's start with P120–P150. Sand perpendicular to the layer direction so that ridges aren't "milled in". For large surfaces, definitely use a block.
Common initial mistakes (and how we avoid them)
- Too much pressure: generates heat, PLA/PETG smudges → gentle pressure, block, water.
- Skipped grits: the coarse scratches remain → only move to each grit once 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/cracks thinly, leave slightly raised. After curing, level with P320–P400. Several thin applications 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 ensure 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. (See MatterHackers, All3DP)
Source: 33d.ch
Spray thinly, let dry, sand wet. Mark problem areas and touch up specifically – saves time.
Step 3: Paint build-up – adhesion before appearance
3.1 Primer → paint → clear coat
- Primer (grey): final adhesion and control layer. Reveals remaining flaws.
- Paint: 2–3 thin coats instead of one thick one. For airbrush: several fine mist coats.
- Clear coat: matte/satin/gloss – protects and adds depth.
Proven practice in many workshops and guides – and standard for us too. ( (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 will occur.
- Drying times: Filler surface dry 20–60 min, fully cured several hours (follow manufacturer's instructions).
Defect → Cause → Solution (Table)
| Defect | Probable Cause | Practical Solution |
|---|---|---|
| Orange Peel | Sprayed too thick, too close, cold can | Spray thinner, 20–30 cm distance, warm can (hand warm), wet sand P600 |
| Bubbles/Fisheyes | Grease/dust, silicone residue | Degrease thoroughly (IPA), work dust-free, thin coats |
| Visible sanding marks | Skipped grits | Go back to the last grit until all scratches are gone; then proceed |
| Flaking paint | Primer missing/inadequate | Use adhesion promoter/primer, sand surface with P600, check cleaner |
| Smudgy sanding | PLA/PETG heats up | Wet sand, 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.
- Coarse sanding: completely P150 → P220, until everything is uniformly matte.
- Putty: fill seams/pores thinly, level with P320.
- Filler cycle: spray thinly → dry → wet sand P400–P600. Two rounds.
- Primer (grey): Control sanding P800 wet.
- Paint: 3 thin coats (e.g., skin tone, clothing via airbrush, details with brush).
- Clear coat: 2 coats (desired gloss). Optional polishing with P2000+ and compound.
Source: 33d.ch
After two filler cycles and clean intermediate sanding, the figure looks "as if cast", details are preserved.
PLA vs. PETG – small differences that make a big difference
- PLA: can be filled and painted very well. Be careful with heat during sanding.
- PETG: tougher, can clog paper, adhesion trickier. We sand rougher (P320) and often use adhesion promoter before filler/primer.
- Epoxy coating: for extreme smoothness/robustness possible; higher effort. (Overview at (Fictiv)
Insights from our workshop (33d.ch)
- "Marker pen check": After the first filler application, we spray dark lines on problem areas and then sand them specifically. Saves time.
- "Thin is winning": Thick coats 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)
- Clean raw part, break edges/support scars with P120–P150
- Completely P220 → P320/400, until uniformly matte
- Apply putty locally, level with P320/400
- Filler thinly → wet P400/600 → repeat if necessary
- Primer grey → control sanding P800
- Paint 2–3 thin coats
- Clear coat as desired, optional polishing
Recommended Resources (Backlinks)
- All3DP: PLA glätten – Methoden & Praxis
- MatterHackers: PLA schleifen, füllern, lackieren
- Prusa Blog: Post-Processing Schritt für Schritt
- 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
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.
- Marking, checking, and targeted touch-ups save around 20–30% of finishing time.
Note: Settings are examples – printers/materials vary. Test on sample parts first.