Post-processing for Polymer AM: SLS, MJF, and FDM Surface Finishing Guide

A polymer AM part leaving the machine is rarely the finished product. SLS and MJF parts are coated in sintered powder cake. FDM parts have visible layer lines. SLA parts are still photoreactive and mechanically under-developed. Each process has its own starting condition and its own post-processing menu. This guide covers what each step does, what equipment it requires, and what results to expect.

SLS and MJF starting condition

SLS (Selective Laser Sintering) and MJF (Multi Jet Fusion) parts emerge from the powder bed embedded in a cake of partially sintered or fused powder. Before any surface treatment is possible, the powder must be removed.

As-built surface condition: Ra 10–15 µm on external surfaces. This is rougher than most machined polymer surfaces but finer than FDM. Colour is uniformly grey or off-white (natural PA12). Internal channels and cavities contain loose powder that must be removed.

Powder cake removal sequence:

Manual brushing — remove the bulk of the loose cake
Compressed air blow-off — clear loose powder from surfaces and shallow channels
Bead blasting — this is standard, not optional, for most SLS/MJF workflows; it simultaneously cleans residual powder and improves surface finish

Depowdering techniques

Manual brush and compressed air

The first stage of depowdering for all SLS/MJF parts. Requires no special equipment beyond a standard blast cabinet or manual workstation. Limitations: labour-intensive for large batches; ineffective for internal channels; inconsistent between operators.

Automated vibration (Solukon, DyeMansion)

Automated depowdering stations (Solukon SPR-Spin, DyeMansion PowderToSurface) rotate and vibrate the part in programmable motion sequences while applying compressed air jets through computer-controlled nozzles. The motion sequence is designed to drain powder from cavities at all orientations.

Key advantage: internal channel depowdering that compressed air alone cannot achieve. Critical for any part with blind or tortuous internal passages — fluid channels, medical devices, wearables with internal structure.

For complex internal geometries, the part orientation and the nozzle programme must be specified at the design stage. If powder cannot drain from an internal cavity during automated depowdering, it will be permanently trapped.

Ultrasonic cleaning

For fine internal channels (< 2 mm diameter) in SLS parts, ultrasonic baths with water and mild detergent can dislodge powder that automated vibration cannot reach. The ultrasonic cavitation breaks up sintered agglomerates. The part must then be thoroughly dried — residual moisture in a nylon part will affect mechanical properties and potentially cause issues during subsequent dyeing or coating.

SLS and MJF surface improvement

Once the part is depowdered, several surface improvement options are available, each with different results:

Shot/bead blasting

The universal first step for all SLS/MJF parts. Plastic bead (typically polyamide or glass bead) at 2–4 bar pressure:

Ra → 6–8 µm from 10–15 µm as-built
Closes partially sintered surface particles, giving a uniform matte white finish
Removes grey residue from MJF detailing agent on natural PA12 parts
Very low cost; standard in all SLS/MJF bureaus

The resulting finish is suitable for many functional applications. Fatigue performance is slightly improved by the compressive surface stress induced by blasting — relevant for snap-fit and spring features.

Barrel tumbling (mass finishing)

Parts placed in a tumbling drum with abrasive media (ceramic or plastic chips) for 2–8 hours. Gentler than blasting; better at edge rounding. Achieves Ra 3–5 µm.

Better than blasting for: small intricate parts, medical devices, consumer goods where edge sharpness is undesirable. Worse than blasting for: thin walls, delicate features that could fracture in the tumbling action.

The media type determines the aggressiveness. Hard ceramic media cuts faster; soft plastic media polishes without cutting.

DyeMansion PowerFuse (vapour smoothing)

DyeMansion's PowerFuse S uses a solvent vapour atmosphere to melt the outer skin of the part surface. The skin reflows into a smooth layer, dramatically reducing Ra.

Result: Ra 0.5–1.5 µm — approaching injection-moulded appearance. This is a step-change improvement over blasting.

What PowerFuse does and does not do:

Does reduce roughness of outer surfaces dramatically
Does improve visual appearance and tactile feel
Does seal the outer surface, slightly reducing moisture absorption
Does NOT improve dimensional accuracy — the reflow adds a small amount to outer dimensions (0.05–0.1 mm typically)
Does NOT significantly improve mechanical properties
Does NOT work on internal channels (vapour acts only on accessible outer surfaces)

The vapour smoothed surface has a slight gloss or semi-gloss appearance, not the matte texture of blasted PA12. For consumer-facing parts this is often desirable; for functional parts requiring matte finish, additional surface treatment may be needed.

HP 3D Latex coating

Available on HP MJF systems, HP's 3D Latex coating is a liquid coating applied to blasted MJF parts to provide colour, UV stability, and improved surface sealing. Applied in bulk in an automated coater.

Suitable for: functional coloured parts, wearables, industrial parts requiring UV resistance for outdoor use. Not suitable for: precision-tolerance parts (adds 0.1–0.2 mm), parts requiring electrical conductivity or EMI shielding.

Dyeing of PA12 and PA11

Nylon (PA12, PA11) dyes readily with reactive textile dyes — a major cosmetic advantage over polymer processes that require paint or coating for colour. Dyeing is a bulk process: many parts can be dyed in the same batch.

Process: Parts are immersed in a heated dye bath (80–95°C) with reactive dye (Rit DyeMore for small batches; industrial reactive dye for bureau-scale production) for 20–40 minutes. Rinsed in clean water, dried.

Results:

Uniform colour penetration to 0.2–0.5 mm depth
Colour is embedded in the nylon polymer structure, not a surface film — it does not peel, flake, or chip
Full range of standard textile colours available
Does not affect dimensional accuracy

Colour fastness considerations:

PA12 dye is reasonably UV-stable for indoor applications but will fade with prolonged UV exposure
Wash fastness is generally good for most reactive dyes
Light colours (white, yellow, pale blue) are harder to achieve — dyeing deposits colour additively; it cannot lighten a grey part
Most SLS/MJF bureaus offer black dyeing as a standard finish. Custom colours require larger batch sizes to justify bath setup

MJF colour parts: HP MJF can produce full-colour parts directly using printing agents. Post-process dyeing is used when uniform single colour over a large batch is more cost-effective than printing colour on each part individually.

FDM surface finishing

FDM parts start with visible layer lines (Ra 15–50 µm depending on layer height). Unlike SLS/MJF, there is no powder to remove, but the layer line texture is usually the primary cosmetic and functional concern.

Sanding

The most accessible FDM finishing method. Wet sand with progressively finer grits:

120 grit — removes visible layer ridges
220 grit — smooths the reduced ridges
400 grit wet — semi-smooth surface
800–1200 grit wet — near-smooth for paint adhesion or cosmetic finish

Time: 20–60 minutes of manual sanding for a small-to-medium part. Effective but labour-intensive. The main risk: material removal is uneven, particularly on curved surfaces; thin walls and fine features can be sanded through.

After sanding to 400 grit, a spray primer can be applied to fill remaining micro-texture, sanded again at 400–800, then topcoated.

Acetone vapour smoothing (ABS only)

Acetone vapour dissolves and reflows the outer surface of ABS, producing a glossy, near-smooth finish with layer lines largely invisible.

Strictly for ABS. Acetone does not affect PLA, PETG, ASA, PEEK, or any non-ABS material. Applying acetone to PLA produces no effect. Applying it incorrectly can cause excessive dissolution and part distortion.

Process: parts are suspended over a small acetone reservoir in a sealed container. Acetone vapour rises and condenses on the part surface. 10–30 minutes exposure is typical; longer exposure causes loss of fine detail and dimensional distortion.

Safety: acetone vapour is highly flammable. Process must be conducted away from ignition sources. Commercial vapour smoothing enclosures (Polymaker Polysher) provide a controlled, safer environment.

Result: Ra < 2 µm, glossy surface. Best for aesthetic consumer parts, prototype housings, display models.

Polyurethane primer and paint

Standard automotive finishing workflow applied to FDM:

Sanding to 220–400 grit
Spray apply filler/high-build primer
Sand primer at 400–600 grit
Topcoat (lacquer, enamel, or rattle-can acrylic)

Produces any colour or finish. The primer fills remaining surface texture. Compatible with all FDM materials (PLA, ABS, PETG, ASA, nylon, PEEK) — unlike acetone smoothing, there is no material compatibility issue.

Time and cost: primer + paint adds 1–2 hours of work per part. The paint adds 0.1–0.3 mm to outer dimensions and is not suitable for precision mating surfaces.

XTC-3D epoxy coating

XTC-3D (Smooth-On) is a two-part epoxy specifically designed for FDM surface finishing. Mixed and brushed or rolled onto the part surface; self-levels and fills layer lines. Cure time 4–6 hours.

Result: hard, glossy shell; layer lines effectively eliminated; structural stiffness slightly increased. The coating adds 0.5–1 mm to outer dimensions (more than painting). It can be sanded and painted over.

Food-safe formulations are available (FDA-compliant) — relevant for kitchen tools and food containers printed in food-safe PLA or PETG.

XTC-3D is a reasonable one-step option when speed matters: no spray equipment required, and the self-levelling action does most of the work.

SLA and DLP post-curing

SLA and DLP parts are not fully cured when they leave the printer. The photopolymer is partially polymerised — mechanically weak, dimensionally unstable, and still reactive to UV. Post-processing is mandatory.

Wash station

Residual uncured resin on the surface must be removed before post-curing. Options:

IPA (isopropyl alcohol): Standard wash solvent. 2× 5-minute washes in separate containers, agitated. 99% IPA is preferred.
Formlabs Form Wash: Automated agitation in IPA. Reduces operator solvent exposure.
Tripropylene glycol monomethyl ether (TPM): Lower VOC alternative to IPA; preferred in jurisdictions with strict VOC regulations. Formlabs' official recommendation for newer resins.

The washed part should be visually clean with no tacky surface. Residual resin on the surface will cure as a film during post-cure, causing surface inconsistencies.

Wash solvent saturation with uncured resin eventually requires disposal. Saturated wash IPA can be recovered by UV-curing the dissolved resin (making it solid) and filtering before disposal, reducing hazardous waste volume.

UV post-curing

Parts are placed in a UV post-curing station (405 nm UV lamps, rotating platform for even exposure) for a minimum of 30 minutes. Temperature-assisted post-cure improves mechanical properties significantly.

Effect of temperature during post-cure: Heated post-cure chambers (Formlabs Form Cure, Phrozen Cure, third-party options) at 40–80°C improve final conversion of unreacted monomers and produce:

Higher tensile strength and hardness
Better thermal resistance (HDT — heat deflection temperature)
More predictable and stable mechanical properties

Minimum post-cure recommendation: 30 min at 405 nm, 60°C. Extended post-cure (2+ hours) does not improve most resins significantly after the primary cure window and can cause yellowing in clear resins.

Material-specific notes:

Tough resins: longer post-cure (60–90 min) improves toughness
Flexible resins: avoid over-curing — extended UV exposure makes flexible resins brittle
Clear resins: post-cure in water bath to minimise yellowing (UV absorption by water reduces thermal effects)
Engineering resins (Formlabs High Temp, Rigid, etc.): follow the manufacturer's specific post-cure protocol exactly — these are optimised

Functional coatings

Beyond surface finish, several functional coatings extend the performance of polymer AM parts:

EMI shielding

Conductive spray coatings (silver, nickel, or copper-loaded) applied to the inner or outer surface of polymer housings provide electromagnetic interference shielding. Relevant for electronics enclosures, antenna covers, and test fixtures.

Application: spray or brush after surface sanding/priming. Typical coating thickness 10–40 µm. Achieves shielding effectiveness of 40–60 dB at frequencies relevant for commercial electronics (1–10 GHz) depending on coating thickness and uniformity.

Surface conductivity measurement: 4-point probe resistance measurement verifies coating continuity. Discontinuities in the coating degrade shielding effectiveness sharply.

Dielectric coatings

Parylene conformal coating (CVD process, vacuum chamber required) provides a thin, pinhole-free dielectric coating. Primarily used for SLA/DLP parts that serve as structural carriers for electronics, where the polymer surface must be sealed against moisture ingress.

Parylene C (0.5–25 µm) is the most common variant. It is transparent, biocompatible, and provides excellent moisture barrier and dielectric properties.

Anti-static (ESD-safe PA12)

For SLS PA12 parts used in electronics handling, ESD-safe variants of PA12 powder (carbon black or carbon fibre filled) produce parts with volume resistivity of 10⁶–10⁹ Ω·cm — appropriate for ESD protection. These materials are available as standard feedstocks from EOS, Sinterit, and powder suppliers.

No coating is required; the ESD property is intrinsic to the material. However, post-processing (blasting, dyeing, vapour smoothing) can alter the surface resistivity and should be validated if ESD compliance must be maintained after post-processing.

Quality checks after post-processing

Post-processing changes dimensions, surface condition, and in some cases mechanical properties. Verification is needed before parts enter service.

Dimensional verification

Coatings and finishes add measurable material:

Bead blasting: < 0.05 mm change (abrasive removes material)
Vapour smoothing: +0.05–0.1 mm (surface reflow adds material)
XTC-3D epoxy: +0.3–1.0 mm (significant — must be accounted for in design)
Paint primer + topcoat: +0.1–0.3 mm

For parts with dimensional tolerances tighter than ±0.3 mm, measure after every process step. For loose-tolerance parts, verify critical interfaces (snap-fit slots, bearing bores, pin holes) only.

Colour consistency

Dye batches vary in colour saturation and evenness if temperature or time is not controlled precisely. For consumer-facing parts requiring consistent colour: maintain dye bath temperature ±2°C, use a thermocouple (not guesswork), replace the dye bath after a defined number of parts.

Colour matching to Pantone or RAL standards requires calibrated reference panels and spectrophotometer measurement. Colour-by-eye matching across batches will drift.

Surface contamination check

After post-processing, contamination from: dye residue, blast media fragments, vapour smoothing solvent, coating overspray, or ultrasonic cleaning bath chemistry can remain on the part surface.

Inspection: visual under good lighting; lint-free wipe test; for medical devices, ICP-MS or XRF trace element analysis of the surface may be required.

Rinse in clean DI water and dry after any wet process (dyeing, ultrasonic cleaning). Inspect under raking light (at a low angle) to reveal surface contamination that is invisible under direct overhead light.

Related tools

Surface roughness reference — Ra by process, condition, and post-processing step
Surface treatment selector — choose the right surface treatment for your polymer process and application
Cost per part calculator — model total part cost including post-processing labour and material
Post-processing time estimator — estimate time for each post-processing step