Finishing guide

Spraying 3D Printing Resin: A Simple Tutorial

SLA and DLP parts come off the build plate smooth but not paintable. This tutorial walks through washing, curing, sanding, priming and spraying 3D printing resin parts so the coating actually sticks. Written for engineers and model makers who need a repeatable process, not a one-off lucky result.

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SLA / DLP / LCDGrit 400–20002K primer24 h full cure

Spraying 3D printing resin on an SLA part in a spray booth

On this page

1
Key takeaways
2
Why spraying 3D printing resin needs its own process
3
Washing, curing and support removal
4
Sanding sequence for a paintable surface
5
Primer choice and how to apply the topcoat
6
Step by step: spraying 3D printing resin parts
7
Which primer and topcoat for which part
8
Common defects and what causes them

Quick answer

Key takeaways

Cure before you sandUncured resin clogs paper and smears. Full post-cure first, then sand.

Start at 400 gritWet sand 400 → 800 → 1000 → 2000. Skip a step and scratches show through the paint.

Primer is not optionalResin is non-porous. A 2K or epoxy primer gives the topcoat something to bite.

Thin coats, 150–200 mmTwo or three light passes beat one wet coat. Runs are hard to sand out.

Wait 24 h before handling2K topcoats stay soft for hours. Full hardness takes about a day.

Why resin behaves differently

Why spraying 3D printing resin needs its own process

Resin prints are not like FDM parts. There are no layer ridges to bury under filler, and the surface is already glossy. That sounds like good news for painting, and it partly is. The catch is that cured photopolymer is smooth and chemically inert. Paint has almost nothing to grip.

A second difference is that the surface is easy to damage. Sanding too hard cuts flats into curved geometry. Solvent-based cleaners can soften the skin. Heat from a hot air gun will warp thin walls in seconds.

So the process is less about hiding defects and more about creating a mechanical key without changing the shape. You remove support nubs, blend the marks, roughen the gloss slightly, then build the coating in thin layers.

If the part is a visual model, the goal is a uniform finish with no orange peel. If it is a functional cover or a housing, the goal is adhesion and scratch resistance. Both use the same steps, but the grit sequence and topcoat choice differ.

1
Smooth surface, poor gripCured resin is non-porous. Abrade it or use a primer with a chemical bond.
2
Heat sensitiveThin sections distort above roughly 60 °C. Keep drying temperatures low.
3
Support marksNubs leave witness marks. Blend them before any primer goes on.

Preparation

Washing, curing and support removal

Start with a clean part. Wash it in two stages: a dirty bath to remove the bulk of the liquid resin, then a clean bath of IPA or the manufacturer's wash solvent for 2–3 minutes. One bath only spreads resin back onto the surface. That residue turns into a sticky film after curing and will lift under paint.

Let the part air dry until the surface is uniformly matte, not patchy. Patches mean solvent is still trapped. Compressed air at low pressure helps, but keep the nozzle 200 mm away so you do not blow solvent into hollow sections.

Post-cure under the recommended lamp or in a cure chamber. Follow the resin datasheet, typically 2–10 minutes per side at 405 nm. Under-cured parts stay slightly soft and sanding tears them. Over-cured parts go brittle and chalky, and the chalkiness shows through thin paint.

Remove supports with flush cutters, then trim the nubs with a sharp blade held almost flat to the surface. Do not pull supports off by hand. It tears craters that need filler, and filler on resin is a separate problem.

1
Two-bath washDirty bath first, clean solvent second. 2–3 minutes in the clean bath.
2
Dry fullyAny shiny patch means trapped solvent. Wait or use low-pressure air.
3
Cure per datasheetUnder-cured tears when sanded. Over-cured goes chalky.
4
Flush cut, then shaveClip the support, shave the nub flat. Never snap it off.

Sanding

Sanding sequence for a paintable surface

Wet sanding is the default for resin. Water carries the dust away and keeps the paper from loading. Add a drop of dish soap to the water to break surface tension. Dry sanding works on small parts but clogs fast, and the dust is a respiratory irritant.

Begin at 400 grit only where you need to remove a mark or a nub. Do not sand the whole part at 400. Flats and edges lose their crispness quickly, and it is hard to bring them back. Use a small block or a sanding stick for flat areas and your fingertip for curves.

Then step up through 800, 1000 and 2000. Each step should remove the scratch pattern of the previous one. Clean the part between grits with water and a soft brush. One stray 400-grit particle left on the surface will carve a line through your topcoat.

The finished surface should be uniformly matte with no shiny spots. Shiny spots are unsanded areas and the most common cause of paint pulling away in patches. If you plan a matte or satin topcoat, stopping at 1000 grit is usually enough. Gloss clear coats want 2000 grit or a light polish.

1
Wet sand with a little soapLess clogging, less dust, longer paper life.
2
Spot-sand at 400Only on marks and nubs, not the whole part.
3
Rinse between gritsA single coarse particle ruins the next step.
4
No shiny spotsShiny means unsanded, and unsanded means poor adhesion.

Primer and topcoat

Primer choice and how to apply the topcoat

Resin needs a primer. A 2K epoxy or 2K urethane primer gives the best adhesion and fills remaining 1000-grit scratches. A standard 1K filler primer works for display models but stays softer and can be lifted by masking tape. Etching primers are made for metal and are not the right choice here.

Spray in light passes. Hold the can or gun 150–200 mm from the surface and move in a steady line past the part. Start the spray before the part and stop after it. Two or three thin coats with 10–15 minutes flash-off between them beat one heavy coat.

Ambient temperature matters more than most people expect. Below about 15 °C, rattle-can propellant drops and the finish goes cloudy. Above 30 °C, solvent flashes off too fast and you get dry spray, a rough sandy texture that has to be sanded back.

Let the primer dry per the datasheet, usually 30–60 minutes before sanding and 24 hours before a topcoat. Then apply the color in the same thin passes. For a gloss finish, a clear coat after 24 hours adds depth and protects the color layer.

1
2K epoxy primerBest adhesion on resin. Fills 1000-grit scratches.
2
150–200 mm distanceCloser causes runs. Further causes dry, sandy overspray.
3
15–30 °C windowCold finish clouds. Hot finish dries before it lands.
4
24 h before topcoatPrimer keeps shrinking for hours. Painting early traps solvent.

Process

Step by step: spraying 3D printing resin parts

Follow the order. Each step assumes the previous one is finished and dry.

1
Wash in two bathsDirty IPA bath 2 minutes, clean bath 2–3 minutes. Agitate gently with a soft brush. Do not leave the part soaking for hours; solvent softens the surface.
2
Dry and inspectAir dry until matte. Look for white residue in recesses. If you find any, go back to the clean bath for 30 seconds.
3
Post-cure fully2–10 minutes per side under the recommended lamp, or follow the resin datasheet. The part should feel hard and sound slightly dull when tapped.
4
Remove supports and shave nubsFlush cutters first, then a blade held nearly flat. Blend the mark with a 400-grit stick in one direction, not in circles.
5
Wet sand 400 → 2000Spot-sand at 400, then full passes at 800, 1000 and 2000. Rinse with water and a soft brush between each grit.
6
Degrease and dryWash with warm water and a mild detergent, rinse, then dry with clean compressed air. Handle the part with nitrile gloves from here on. Skin oil is a common cause of fisheyes.
7
Prime in thin coats150–200 mm distance, two or three passes, 10–15 minutes flash-off. Let it dry 30–60 minutes, then wet sand lightly at 1000 grit if you need a flatter surface.
8
Topcoat and cureSame distance and pass technique. Keep coats thin. Let the part sit 24 hours at room temperature before handling or assembly.

Selection

Which primer and topcoat for which part

Match the coating system to what the part has to survive.

Part type	Primer	Sand to	Topcoat
Display model, indoor	1K filler primer	1000 grit	Acrylic color + satin clear
Handled housing	2K epoxy primer	1000 grit	2K urethane, matte
Outdoor enclosure	2K epoxy primer	800 grit	2K urethane + UV clear
Functional cover, wear	2K epoxy primer	800 grit	2K polyurethane, semi-gloss
Clear or tinted part	Adhesion promoter only	2000 grit	2K clear, thin coats
Fine detail, thin walls	Thin 1K primer	1000 grit	Light mist coats only

Troubleshooting

Common defects and what causes them

Symptom	Cause	Fix
Paint pulls off in patches	Sanded surface still glossy	Re-sand at 800, re-prime
Small craters or fisheyes	Skin oil or silicone on part	Degrease, wear nitrile gloves
Sandy, rough texture	Dry spray, too far or too hot	Move to 150 mm, cool the booth
Runs and sags	Coat applied too wet	Sand flat at 1000, recoat thinner
Cloudy or milky finish	Cold propellant or damp air	Spray above 15 °C, lower humidity
Fine scratches visible	Grit step skipped	Repeat 800 → 1000 → 2000 sequence
Topcoat stays soft	Primer not fully dry	Wait 24 h before color coat

The short version

Wash it twice, cure it fully, sand from 400 to 2000, prime in thin coats, and give the topcoat a full day. Skip any one of those and the finish fails at the worst time.

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FAQs

Frequently asked questions

Can I spray resin parts without sanding?

You can, but the coating will flake off under any handling. The gloss surface offers almost no mechanical key.

If sanding is impossible because the detail is too fine, use an adhesion promoter made for plastics and keep the topcoat thin. It works better than nothing, but it is not as durable as a sanded and primed surface.

What grit should I stop at before priming?

For a matte or satin finish, 1000 grit is enough. The primer fills the remaining scratch pattern.

For a high-gloss clear coat, go to 2000 grit and consider a light polish. Anything coarser will show as fine lines under the gloss.

Why does the paint look rough and sandy?

That is dry spray. The solvent evaporated before the droplet reached the part, so the resin landed as partly dried particles.

Move the can closer, to about 150 mm, slow your pass, and check the booth temperature. Above 30 °C this happens fast.

How long before I can handle the finished part?

Wait 24 hours at room temperature for a 2K topcoat. It feels dry in an hour but stays soft underneath.

If you need to assemble sooner, use a low-temperature oven cure only if the resin datasheet allows it. Above roughly 60 °C thin resin walls can distort.

Does spraying 3D printing resin work on hollow parts?

Yes, as long as drain holes are clear and the inside is fully cured. Trapped solvent or uncured resin will outgas and push through the paint as small blisters.

Wash hollow parts longer, drain them thoroughly, and post-cure with the holes facing down so air can move through.

Can I use the same process on FDM parts?

The washing and curing steps do not apply, but sanding and priming do. FDM parts have layer lines, so you usually need filler primer or spot putty first.

The coating choice is the same. 2K epoxy primer still gives the best adhesion on PLA, PETG and ABS.