Surface finishing 3D printed parts is the gap between a raw print and a production-ready component. Every 3D printing process produces characteristic surface artefacts — FDM leaves visible layer lines (Ra 15–30 µm), SLS leaves a grainy powder texture (Ra 10–15 µm), and SLA leaves subtle print lines and support witness marks (Ra 1–4 µm). With appropriate post-processing, all three processes can achieve smooth, painted, plated, or anodised surfaces indistinguishable from injection moulding or machined metal parts. According to Formlabs' 2024 post-processing survey, 67% of professional 3D printing users cited surface finish as the primary barrier to using printed parts as end-use products — yet 80% of those cases were resolved by adding a post-processing step. At Layer X, we offer media blasting, dyeing, painting, and electroplating as part of our standard service. This guide covers every surface finishing technique from manual sanding to industrial vapour smoothing.
SLS Nylon: The Standard Finishing Workflow
SLS parts exit the build chamber covered in unsintered powder — the first step is always powder removal and media blasting. Standard SLS finishing workflow at Layer X: (1) Depowdering — compressed air removes loose powder from external surfaces and accessible cavities. (2) Media blasting — glass bead or aluminium oxide blasting removes remaining surface powder, reduces Ra from 15 µm to 7–8 µm, and creates a consistent matte surface. This step is included in every SLS order. (3) Optional dyeing — SLS PA12 accepts reactive dye in black, grey, or custom colours (RAL chart available). Dye penetrates 0.3–0.5 mm into the surface, so scratches reveal undyed material beneath. (4) Optional primer + paint — 2K polyurethane primer applied after blasting, sanded to 320 grit, then topcoat in any RAL colour. (5) Optional vapour smoothing — acetone or proprietary solvent vapour treatment melts the surface layer, flowing and smoothing from Ra 8 µm to Ra 1.5–2.5 µm in a single automated step.
| Finishing Step | Starting Ra | Resulting Ra | Adds Cost | Best For |
|---|---|---|---|---|
| Media blast (standard) | 15 µm | 7–8 µm | Included | All SLS parts |
| Dye (black) | 7–8 µm | 7–8 µm | Low | Cosmetic colour parts |
| Primer + 2K paint | 7–8 µm | 1.5–3 µm | Medium | Cosmetic production parts |
| Vapour smoothing | 7–8 µm | 1.5–2.5 µm | Medium | Sealing surfaces, cosmetic parts |
| Electroplating (copper/nickel) | 7–8 µm | 0.5–1.5 µm | High | Conductive, metallic appearance |
FDM: Sanding, Filling, and Priming
FDM parts have visible layer lines (Ra 15–30 µm) that are most obvious on angled surfaces and curved forms. Finishing FDM requires physical material removal to eliminate layer steps. Manual sanding workflow: Start at 120 grit (removes gross layer lines), progress through 180, 220, 320, 400, and finish at 600–800 grit for a smooth base. Work wet (water with a drop of dish soap) from 320 grit upward — this prevents heat buildup that deforms the print. After 400 grit, apply a high-build automotive filler primer (Motip or Spies Hecker) in 2–3 thin coats, allow to cure 30 min, then block-sand with 400 grit flat paper. Repeat primer + sand until surface is perfectly smooth and uniform — typically 2–3 cycles. Topcoat in any automotive 2K paint or powder coat. For ABS, acetone vapour smoothing is an alternative to sanding — acetone dissolves ABS's surface layer, flowing it smooth in minutes. Do not apply acetone to PLA, PETG, or ASA — they are not acetone-soluble.
Spray filler primer for 3D printing: Tamiya Surface Primer (fine, for small detail parts), Motip Plastics Primer (adhesion promoter for polypropylene-like surfaces), or professional 2K automotive high-build primer (for large production runs). Apply in thin coats at 40–50 cm distance — thick coats sag and trap solvent, causing adhesion failure.
SLA Resin: The Best Starting Surface
SLA resin parts have the best as-built surface of any polymer process — Ra 1–4 µm — but require UV post-cure and support removal before any finishing. Support removal: use flush cutters to clip supports, then sand witness marks with 220 grit. The resin is brittle — apply lateral pressure, not twisting force, to avoid cracking the part. UV post-cure for 30–60 minutes under 405 nm light solidifies any unreacted resin and dramatically improves surface hardness and chemical resistance — non-negotiable before painting. After post-cure, SLA resin can be sanded (220→400→600→800 grit) to near optical smoothness. For clear/transparent parts: sand to 2,000 grit, then polish with Novus #2 plastic polish and microfibre cloth to achieve optical clarity comparable to cast acrylic. For painted cosmetic parts: after 600 grit, apply adhesion promoter (important — resin is chemically inert and paint can peel without it), then standard automotive primer + 2K topcoat.
Metal Parts: Electropolishing and Anodising
DMLS metal parts benefit from electropolishing (for stainless and titanium) or anodising (for aluminium) as the primary surface finishing steps. Electropolishing of 316L stainless: Electrochemical surface dissolution removes Ra 10 µm peaks, producing Ra 0.4–0.8 µm surfaces that are highly corrosion-resistant, easy to clean, and hygienic. Essential for food-contact, pharmaceutical, and medical parts. Electropolishing also reveals subsurface defects — cracks and large pores that were hidden under the rough as-built surface will become visible. Anodising of AlSi10Mg: Electrochemical oxidation of the aluminium surface creates a hard, corrosion-resistant oxide layer. Type II anodise (10–25 µm oxide) is standard; Type III (hard anodise, 25–75 µm) provides higher hardness for wear surfaces. Anodising accepts integral colour — common in aerospace (natural/silver), consumer products (black, blue, red), and automotive. Note: AlSi10Mg's high silicon content (9–11%) can result in slightly blotchy anodise appearance compared to wrought 6061 — this is normal and can be controlled by specifying the correct anodising bath chemistry.
Vapour Smoothing: Automated SLS Finishing
Vapour smoothing for SLS nylon is the most consistent automated finishing method — it eliminates manual sanding entirely and produces a sealed, smooth surface in 60–90 minutes per batch. AMT's PostPro3D machine (used commercially by several Indian AM service bureaus) uses a proprietary chemical vapour (PCMTA or DPF) to dissolve and reflow the top 20–50 µm of the SLS surface. Benefits: Ra drops from 7–8 µm (post-blast) to 1.5–2.5 µm; surface sealed (improved chemical resistance and reduced moisture absorption for PA12); uniform finish regardless of part geometry. Limitations: sharp edges and fine details round slightly (avoid for parts with sharp lettering or very fine features below 0.3 mm); some dimensional change (typically 0.05–0.15 mm reduction per surface — account for this in tight-tolerance designs).
Key Takeaways
- Media blast is always first: All SLS parts are blasted as standard — this reduces Ra from 15 to 7–8 µm and is the baseline for all further finishing.
- FDM needs sanding: Layer lines (Ra 15–30 µm) require progressive wet sanding from 120 to 800 grit + filler primer to achieve a smooth painted surface.
- SLA needs UV post-cure before painting: Non-post-cured resin will not accept paint adhesion — always cure 30–60 min at 405 nm before any surface treatment.
- Electropolish stainless for hygiene: Ra 0.4–0.8 µm surfaces after electropolishing are required for food-contact and pharmaceutical applications.
- Vapour smoothing seals SLS: Automated chemical vapour smoothing reduces Ra to 1.5–2.5 µm and seals the porous SLS surface — ideal for water-resistant cosmetic parts.
Frequently Asked Questions
Can I paint SLS nylon parts in any RAL colour?
Yes. After media blasting and priming, SLS nylon accepts standard 2K polyurethane topcoat in any RAL colour. For production quantities, powder coat is more economical and durable than liquid paint. Layer X can arrange painting and powder coating through partner facilities — specify the RAL colour and finish type (matte, satin, gloss) in your order.
How do I achieve a chrome-like metallic finish on SLS parts?
Two routes: (1) Metallise with physical vapour deposition (PVD chrome or titanium nitride) after primer + paint — produces a genuine metallic surface, 0.5–5 µm thick. (2) Apply chrome spray paint (Spraymax Chrome) over a high-gloss black base — a simulated chrome effect at lower cost, not as durable. PVD metallising is available through electroplating specialists in Mumbai and Bangalore.
Does vapour smoothing change part dimensions?
Yes — typically 0.05–0.15 mm reduction per surface. For most functional parts, this is within the SLS dimensional tolerance (±0.3 mm). For tight-tolerance features, print with 0.1 mm oversize allowance on smooth-sensitive surfaces, or exclude vapour smoothing from those surfaces using masking.
Can DMLS metal parts be painted?
Yes — after media blasting and phosphating (adhesion treatment for steel) or chromating/anodising (for aluminium), DMLS parts accept the same industrial paint systems as machined metal. Powder coat is the most common finish for DMLS steel and aluminium parts. Stainless DMLS parts are rarely painted — electropolishing is the preferred finish.
Why Layer X for Finished Parts?
Layer X provides end-to-end service: print, post-process, inspect, and ship production-ready parts. Our standard SLS includes media blasting. Optional finishes — dyeing, priming, painting, vapour smoothing, and electropolishing — are available with 3–5 day additional lead time. Metal parts include dimensional inspection before any surface treatment. We coordinate electroplating, anodising, and powder coating through certified partner facilities in Ahmedabad and Mumbai. Get your 24-hour quote — specify the required finish and we'll include the complete post-processing workflow in the quotation.