Why parts get sprayed before scanning, and when they shouldn’t be

TL;DR: 3D scanning spray puts a matte white film on a part so the scanner can see it. That film is between about 0.5 and 15 microns thick, which makes it a measurement error you've chosen to add. Modern blue laser and infrared scanners handle a lot of surfaces that used to need coating, so the first question is always whether your part needs it. Some parts should never be sprayed.

3D scanning spray looks like a consumable. Buy a can, coat the part, scan it, job done. Plenty of workshops treat it that way.

It's better understood as a compromise. Every coat sits on top of the real surface, so the thickness of that coat becomes an error in the measurement, deliberately introduced because the alternative was no measurement at all.

That trade is usually worth making. Sometimes it isn't, and sometimes the scanner can manage without it. Here's what the spray does, what it costs you in accuracy, and the parts that shouldn't go anywhere near a can.

What is 3D scanning spray and what does it do?

Scanning spray lays a thin, even white film over a part to make the surface matte and opaque. Optical scanners need light to bounce back evenly. Dark, shiny and clear surfaces don't do that, so the spray gives the scanner a uniform skin to read instead of the real one.

The chemistry varies. Traditional developer sprays deposit fine titanium dioxide or talc powder as a temporary white layer that scatters light evenly in every direction. Newer vanishing sprays skip the pigment and use a compound that sublimates, meaning it turns straight from solid to vapour and leaves nothing behind.

Application matters more than people expect. Coverage should be even and light, applied from around 15 to 20 cm with steady back-and-forth strokes. A thick patch changes the geometry in that patch. A missed patch gives you a hole.

Why dark, shiny and clear surfaces defeat a scanner

Three different failures, all of them about where the projected light goes.

Dark surfaces absorb it. A matte black part takes in most of the light the scanner throws at it and returns very little, so the sensor gets weak data or none. Shiny surfaces do the opposite and throw the light off in one direction, like a mirror, so most of it never reaches the camera and what does arrive lands as glare and false points floating in space.

Clear and translucent parts are the hardest case. The light passes straight through the surface, bounces around inside the material, and comes back from somewhere other than the face you were trying to measure. There's no exposure setting that fixes light going through a part instead of off it.

Our 3D scanning service runs blue light and infrared to 0.04 mm, which handles more of these than older equipment did. It doesn't handle glass.

How much thickness does scanning spray add?

Between roughly 0.5 and 15 microns for purpose-made sprays. Fine metrology versions sit at about 0.5 to 1 micron, general-purpose ones at 8 to 15, and large-area products around 4. Pigment sprays applied heavily can add tens of microns, unevenly.

Those figures come from the products themselves. One widely used metrology spray adds approximately 0.5 to 1 micron, the general-purpose version in the same family adds around 8 to 15 microns, and the large-object version sits at about 4 microns.

Whether that matters depends entirely on your tolerance. On a bracket held to a quarter of a millimetre, 15 microns is noise. On an inspection job, it isn't: when you're working to a 25 to 50 micron tolerance band, an uncontrolled coat can swallow the whole band before the scanner sees the part, and because the thickness varies from spot to spot it can't be corrected afterwards in software.

Which is why the instinct to spray generously is the wrong one. A thicker coating lasts longer and costs you accuracy, and clear or mirrored surfaces need full coverage while dark or reflective ones only need a light pass.

Vanishing spray against developer spray

Two families, and the difference is what happens after the scan.

Developer sprays are pigment-based and permanent until removed. That means cleaning every coated surface afterwards, which is a nuisance on a simple shape and a real problem on anything with blind holes, fine threads, internal channels or texture, where trapped powder can contaminate assembly or the part's function. They also sit on the surface rather than adhering to it, so moving the part after spraying disturbs the coating.

Vanishing sprays sublimate. The coating clears in around four hours with no cleaning and no pigment left in the room or in air filters, which matters in a lab or near sensitive equipment. Larger-area versions last a little longer, around six hours.

The catch is the clock. Laboratory testing on sublimating coatings found that single-layer coatings started disappearing from the edges of a test wafer after about ten minutes, leaving patchy coverage. Spray, then scan. Don't spray and go to lunch.

Can you scan without spraying?

More often than you'd think. Blue laser and infrared read many dark and shiny surfaces that used to need coating, and exposure settings, scan angle and extra passes handle a good share of the rest. Clear and mirrored surfaces are the ones that still usually need a coating.

Start with the settings. For black parts, raising the exposure to maximum and holding the scanner at a slight angle to the surface recovers a lot of missing data, because an angled approach spreads the reflection instead of bouncing it straight back. Adjusting sensitivity does similar work, and scanning at a slight angle and distance is the first thing to try on anything glossy.

Then the hardware. Scanners combining laser lines with blue LED light capture dark surfaces without markers or white spray, and short-wavelength blue laser systems at around 450 nanometres remove the need for spray on a lot of difficult material.

None of this helps with glass. Light going through a part rather than off it is a physics problem, and coating it is the only answer.

Which parts we won't spray

Anything porous or unfinished, anything going back into service or into a person or a food process, and anything historic. Pigment gets into surfaces it can't come out of, and a scan is never worth ruining the object it was measuring.

The hard stops are specific. Guidance from the scanning trade is blunt about never spraying museum artefacts, historic bones or untreated leather, because pigment permanently stains delicate materials. For food-contact parts, medical items and anything returning to service, residue is a hard stop. Conservators working on archaeological objects use different chemistry again, with cyclododecane used as a coating for scanning archaeological finds.

There's a health angle too. Titanium dioxide in fine powder form carries an EU classification as a suspected carcinogen by inhalation, for mixtures containing 1 per cent or more in particles under 10 microns. Worth knowing what's in the can before someone in your workshop empties one.

Where a part can't be coated, the job changes rather than stops. More passes, different angles, hand measurement on the features that failed, and honest reporting of where the data is thin. That's part of why the model gets checked back against the part before it leaves our product design team.

Two things to hold onto. Spray thickness is measurement error, so match the product to your tolerance rather than grabbing whatever can is nearest. And tell whoever is scanning what the part is made of and where it's going afterwards, because that decides what can be put on it.

Send us the part with a note on its finish and its next destination and we'll tell you how we'd capture it. Email sales@nexform.co.uk or use the contact form with a CAD file, a sketch or the part itself, and the data goes straight into reverse engineering and on to low volume manufacturing from there.