3D Print Post-Processing: Sanding, Smoothing & Painting Guide
Complete guide to 3D print post-processing: grit-by-grit sanding, ABS acetone vapor smoothing, XTC-3D epoxy coating, priming and painting. Separate section on washing and UV curing SLA/MSLA resin prints.
Your print just came off the printer looking decent. After post-processing it'll look professional. Layer lines will disappear, surfaces will be smooth, paint will apply evenly. This guide covers everything you need to know about finishing both FDM and resin (SLA/MSLA) prints.
Sanding: Removing Layer Lines
Sanding works for all plastics (PLA, ABS, PETG, resin) and requires no chemicals. It's the starting point for any subsequent finishing.
Sanding is the most universal post-processing method. The principle is simple: work from coarse grit (removing layers) to fine grit (polishing). Golden rule: never skip grit levels — scratches from 120 grit won't disappear after 600 grit, you need an intermediate 220 or 320 step.
ABS removes faster than PLA: removing just 0.010" (0.25mm) eliminates visible stepping entirely. Wet sanding is especially critical for ABS — heat deforms the part.
PETG sticks to sandpaper when dry. Use wet sanding from 220 grit. Circular motions are mandatory — straight-line sanding emphasizes layer lines instead of removing them.
Grit sequence: 220 → 320–400 → 600–800 → 1000–2000 (mirror: up to 12000)
Always wet sand resin! Dry sanding creates toxic and carcinogenic dust. Wet the surface, work carefully. Resin prints achieve an incredible mirror-like finish when taken to 2000+ grit.
Wet sanding a resin part — gloves and water are non-negotiable
Wet sanding from 400 grit onward is a rule, not a suggestion. Water controls dust, reduces friction, extends sandpaper life, and prevents plastic from melting from heat.
Acetone Vapor Smoothing for ABS
Acetone works ONLY with ABS and ASA. On PLA it makes the surface sticky and rubbery. PETG doesn't react at all. Don't mix up your materials!
Acetone vapors dissolve the outer ABS layer, fusing the layer lines together — giving you a surface indistinguishable from injection-molded plastic. Fast but requires care. Three methods: cold (room temp, 10–60 min), hot (50–56°C, 10–15 min), and brush application (selective treatment).
Hot method (recommended): pour ~1 cm of acetone into a metal can, suspend your part on a string so it doesn't touch the liquid, heat to 50–55°C. Hold 10–15 minutes, check every 5 minutes. After removal — don't touch for 30 minutes, full hardness restored in 12–24 hours. Research shows 72–81% reduction in surface roughness.
Acetone: flash point −20°C, explosive vapor range 2.6–12.8%. Work ONLY outdoors or with forced ventilation. Nitrile gloves, safety glasses, no open flames. Keep a Class B fire extinguisher nearby.
XTC-3D Epoxy Coating
XTC-3D by Smooth-On is a two-part epoxy coating designed specifically for 3D printing. It fills layer lines, cures to a hard glossy finish, and works with PLA, ABS, PETG, SLA/SLS, foam, and wood. Perfect where acetone isn't an option.
Mix ratio: 2 parts A : 1 part B by volume (or 100A : 42B by weight). Pour the mixed resin onto a paper plate — this extends working time from 10 to ~15 minutes by increasing surface area and reducing heat. Apply with a foam brush in thin coats. Tack-free in 2 hours, full cure in 3.5 hours at 23°C. 1 oz mixed covers 651 cm².
PLA deforms at just 60°C (glass transition). If using heat to accelerate curing, don't exceed 50°C for PLA parts. For PLA, wait 90 minutes between coats instead of the usual 2 hours.
Priming and Painting
Primer is mandatory before painting. It ensures paint adhesion, reveals remaining defects, and evens out the surface. Filler/sandable primer is the best choice for FDM prints: it's thicker than standard primer, fills fine scratches, and sands easily.
Process: apply 2 coats of primer at 15–20 cm distance with quick passes. Let dry, wet sand with 500–600 grit, re-prime if needed. Then acrylic paint in 2–4 thin coats with 20–30 minutes drying between layers. Finish with 1–2 coats of clear coat (matte/gloss/satin). After clear coat, let the model rest at least one week before heavy use.
Recommended primers: Tamiya Surface Primer (ultra-thin, ideal for miniatures), Vallejo Surface Primer (airbrush-ready), Rust-Oleum Filler Sandable (great for FDM/PLA). Avoid nitro enamels and alkyd primers — they can attack some plastics. Stick to acrylics.
Resin Post-Processing (SLA / MSLA / DLP)
Uncured resin is toxic and carcinogenic. Nitrile gloves are mandatory at all stages before final UV curing. Everything that contacts liquid resin (containers, cloths, gloves) must be disposed of as chemical waste.
Resin printing requires a specific workflow. A freshly printed part hasn't reached full strength and is coated in liquid resin. The correct sequence is critical.
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Specialized cleaners like Liqcreate Resin Cleaner — a less aggressive IPA alternative
Use 99% IPA — lower concentrations leave white streaks. Two-bath system: first 'dirty' bath removes bulk resin, second 'clean' bath does the final rinse. Maximum wash time: 20 minutes, any longer and the part will swell and crack.
What Works with Which Material
Material
Sanding
Acetone
XTC-3D
Painting
PLA
✓ 120–1000 grit
✗ Doesn't work
✓ Excellent
✓ Primer required
ABS
✓ 100–1000 grit
✓✓ Best method
✓ Works
✓ Acrylics
ASA
✓ 100–1000 grit
✓✓ Like ABS
✓ Works
✓ Acrylics
PETG
✓ 100–1000 (wet!)
✗ Ineffective
✓ Works
✓ Acrylics
Resin SLA
✓ 220–2000+ (wet only!)
✗ N/A
✓ After curing
✓ After curing
Nylon
✓ 120–600 grit
✗ No reaction
✓ Epoxy
~ Difficult
PLA
Sanding: ✓ 120–1000 grit · Acetone: ✗ Doesn't work · XTC-3D: ✓ Excellent · Painting: ✓ Primer required
ABS
Sanding: ✓ 100–1000 grit · Acetone: ✓✓ Best method · XTC-3D: ✓ Works · Painting: ✓ Acrylics
ASA
Sanding: ✓ 100–1000 grit · Acetone: ✓✓ Like ABS · XTC-3D: ✓ Works · Painting: ✓ Acrylics
Post-processing turns a 'printed part' into a 'finished product.' Start with sanding — it works for all materials and sets the foundation for every next step. For ABS, add acetone smoothing. For PLA, epoxy coating. Priming and painting finish any model regardless of printing technology.
Only sanding — acetone doesn't work on PLA (leaves it tacky). Work from coarse grit (P120, removes layers) to fine (P600–1500, polish). Never skip steps: P120 scratches don't vanish after P600 without intermediate P220/P320. Wet sanding from P400 up is mandatory: water holds dust, reduces friction, and prevents the plastic from melting from heat.
Hot method: pour ~1 cm of acetone into a metal can, suspend the part on a string above the liquid, heat to 50–55°C in a water bath. Hold 10–15 minutes, check every 5 minutes. Remove and don't touch for 30 minutes. Full strength returns in 12–24 hours. Studies show surface roughness reduction of 72–81%. WARNING: acetone is flammable, work outdoors or under a fume hood.
No, PETG doesn't react to acetone. For PETG smoothing, use only sanding (P220 → P400 → P800 → P1500) or chemical solvents dichloromethane or methyl ethyl ketone (MEK) — both toxic, only under a fume hood with a respirator. Acetone doesn't work on PLA either — leaves it tacky. Acetone is ONLY for ABS and ASA. Don't confuse materials — this is post-processing fundamentals.
Sequence: sand (P220→P400→P600) → primer (2 coats gray) → sand primer P800 → paint with acrylic enamels in 2–3 coats. Without primer, layers still show through paint. Filler primer is better — it smooths small imperfections. For spray cans, hold 25–30 cm away with thin coats and 15 minutes drying between them. Finish with a protective clear coat.
SLA/MSLA models wash in 90%+ isopropyl alcohol immediately after printing (two 5-minute baths) to remove unreacted resin. Then UV post-cure at 405 nm for 5–15 minutes (resin-dependent). Only after curing can you sand — uncured resin is too soft. Sand the same as FDM but always wet (resin dust causes allergies). Painting uses the same acrylics as FDM.
Mandatory: N95 respirator (plastic microparticles harm lungs) and safety glasses. Dry sanding throws dust — that's why P400+ sanding must be wet. Work at a ventilated table or near a window. For resin, medical gloves are mandatory: uncured resin causes contact allergies. After acetone or MEK work — wash hands and clothes; vapors are flammable, don't smoke nearby.
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