SLS Surface Finish: Options Compared, From Bead Blasting to Vapor Smoothing

An as-printed SLS part has a matte, grainy surface, and you can change it a lot with post-processing. Bead blasting evens out the look but barely lowers roughness. Tumbling smooths it by removing a little material. Vapor smoothing melts the outer skin into a sealed, semi-gloss shell. Dyeing and painting change color and protect the surface, but they do not make it smoother. Which one is right depends on the Ra you must hit, the tolerances you can’t lose and how many parts you need.

This guide compares those options side by side. It also shows how to specify a finish on a quote so you get what you drew.

Key takeaways

• As-printed SLS nylon typically measures Ra 8 to 25 µm, with a sugar-like, matte texture. Treat that as the baseline every finish is compared against.
• Bead blasting cleans parts and makes the texture uniform. It does not meaningfully reduce roughness.
• Vapor smoothing is the only common method here that seals the surface and works on internal and non-line-of-sight geometry. A Formlabs and AMT study report found roughness dropped by about 72 to 81%.
• Every finish that touches the surface moves dimensions a little. Call out critical features and mating faces on the drawing before the finish is chosen.
• Dye for color, paint or coat for protection or a specific look, and smooth first if you want either to look good.

Table of Contents

• What an as-printed SLS surface looks like
• SLS finishing options at a glance
• Media blasting: cleaning, not smoothing
• Vibratory tumbling: the budget route to a smoother part
• Vapor smoothing: the biggest change in surface quality
• Dyeing, painting and coating: color is a separate decision
• How to choose: a decision guide
• Specifying surface finish on a drawing or RFQ
• Our perspective
• Get SLS parts finished the way your drawing says
• FAQ
• Sources

What an as-printed SLS surface looks like

SLS builds parts from fused powder, so the surface is made of partially melted grains. That gives the familiar grainy, matte feel, often compared to medium-grit sandpaper. It also leaves a small amount of open porosity.

Published numbers vary. A 2023 academic review of as-printed polymer roughness puts SLS average roughness (Ra) commonly between 8 and 25 µm, roughly 315 to 985 µin, and notes that some sources cite narrower ranges. Part of the spread is real: powder, laser settings, build orientation and the age of the powder all change the result. Part of it is how roughness is measured. Linear metrics (Ra, Rz) and areal metrics (Sa) are defined by different standards, including ISO 25178 and the ISO 4287 and 21920 series, and they don’t give the same number for the same surface.

Two practical points follow:

1. Orientation matters. Surfaces facing up, down or sideways in the build chamber don’t come out identical. If one face is cosmetic, tell your supplier.
2. Always name the metric. “Ra 3.2 µm” and “Sa 3.2 µm” are not the same requirement. Our guide to Ra specs and how to avoid inspection disputes covers how to write a callout that both sides can measure the same way.

If you need background on the process itself, our overview of SLS process, materials and applications is a good start.

SLS finishing options at a glance

The table below summarizes what each method does for typical nylon parts. Figures marked with a source come from the studies cited in this article. For the rest, treat the description as qualitative and ask for a sample or measured Ra on your own geometry.

Media blasting: cleaning, not smoothing

Blasting is the first step on nearly every SLS part. Glass beads or similar media knock off trapped powder and give the whole surface the same matte texture. That consistency is useful, because as-printed parts can look patchy.

What it does not do is polish. Formlabs describes blasting as a cleaning step that sits ahead of the smoothing techniques. Treat it as prep. If a drawing says Ra 3.2 µm and the part has only been blasted, expect it to fail an inspection.

We cover the related roughness ranges for bead and sand blasting on metals in 32 to 500 µin Ra: bead blasting vs sandblasting. The media and pressure logic carries over, but nylon is far softer than metal, so settings differ.

Vibratory tumbling: the budget route to a smoother part

Tumbling puts parts and abrasive media in a vibrating bowl for hours. Peaks wear down, sharp edges soften and the surface gets a satin feel. It suits robust geometries produced in batches, and it is among the cheaper smoothing methods per part when volumes are decent.

Limits to plan around:

• Material is removed. Thin walls, small holes and fine text can lose definition.
• Edges round over. Sharp corners will not stay sharp.
• Reach is limited. Media can’t get into deep channels or narrow slots, so internal surfaces stay rough.
• Fragile parts can break or deform in the bowl.

If a part has snap-fits, thin ribs or tight press-fit bores, tumble a sample and measure before committing a batch.

Vapor smoothing: the biggest change in surface quality

Vapor smoothing exposes the part to solvent vapor in a controlled chamber. The vapor softens a micro-thin layer, surface tension pulls it flat, and it re-hardens. According to a Formlabs white paper on vapor smoothing SLS parts, the result is a smooth, often semi-glossy shell that fills surface cavities, removes porosity and reaches complex geometry that abrasive methods can’t.

The numbers are significant. A study report from Formlabs and AMT found vapor smoothing cut average SLS roughness by about 72 to 81% and produced a consistent, opaque, semi-gloss finish across Formlabs SLS powders. The same report describes reduced porosity, moisture absorption and bacterial growth, which is why the method shows up in wearables, medical-adjacent housings and consumer products.

What vapor smoothing does and does not do

• It seals. A sealed surface resists dirt, moisture and stains, and it wipes clean.
• It reaches inside. Internal channels and undercuts get treated along with the outside.
• It adds gloss, not a mirror. Expect semi-gloss. It is not the same as optical-grade polishing.
• It can’t fix every defect. Warpage, sink and big surface flaws stay.
• It changes dimensions slightly. The skin is thin, but tight-tolerance features still need a trial.

Our engineers also cover how polishing automation and design rules cut roughness in vapor polishing and optical clarity, which is worth reading if your part is transparent or tightly toleranced.

Equipment and cost: why most teams outsource it

Machine cost is the main reason. Industrial vapor-smoothing systems typically start around $100,000, and a benchtop option like AMT’s PostPro SFX is the smaller alternative. According to the PostPro SFX product listing, that machine supports Nylon 12, with TPU and TPE compatibility planned, and processing takes 90 to 120 minutes per batch.

Per-part service pricing can be low when parts are small and batched. The same Formlabs white paper lists vapor-smoothing charges of $0.98 per rigid orthotic insole (50 parts), $0.49 per eyewear frame (80 parts) and $0.78 per watch strap (50 parts) at a Colorado service bureau. Those are examples of small, repeatable parts in batches, not a price list. Larger parts, one-offs and complex geometry cost more, so ask for a quote on your own file.

For most R&D teams, buying a machine only makes sense at steady production volume. Under that, a prototyping supplier that handles finishing is cheaper than owning the equipment and the solvent handling.

Dyeing, painting and coating: color is a separate decision

SLS nylon is naturally off-white to gray. Teams add color for appearance, identification or to hide wear.

• Dyeing soaks parts in a heated dye bath. It is cost-effective for black or a few standard colors, and it adds almost no thickness. Because the dye penetrates the surface, scratches show less than with paint.
• Painting gives the most color choices, including brand-matched colors, but it needs a prepared surface. Paint over a rough as-printed face and the texture shows through, so sanding, priming or vapor smoothing first matters.
• Coatings such as clear coats or soft-touch layers add a functional or tactile layer. They add film thickness, so account for that on mating parts.

The sequence usually runs: blast, smooth (tumble or vapor), then color. Reversing it wastes the color work.

How to choose: a decision guide

Start from the requirement, not the process.

1. Functional, hidden or internal part (jig, bracket, fixture): leave it as printed, or blast it. Spend nothing on cosmetics.
2. Fit and assembly prototype: blast, and keep the dimensions as printed. Check critical bores after printing.
3. Cosmetic prototype for a design review: vapor smoothing, then dye or paint. This is the closest SLS gets to an injection-molded look.
4. Batch of robust parts in a consistent matte finish: tumble, then dye.
5. Part that must be sealed, wipeable or low-moisture: vapor smoothing. Confirm the material with your supplier first.
6. Optical or Class A appearance: consider whether SLS is the right process. SLA starts with a much smoother surface than SLS, and a machined part can do better still.

If you’re deciding between powder-bed processes, our MJF 3D printing service is another route to compare, and our overview of additive manufacturing types explains the trade-offs.

Specifying surface finish on a drawing or RFQ

Most finish disputes come from vague requirements. A request that says “smooth” can be met in ten different ways. Write down:

• The metric and number: for example, Ra 3.2 µm, with the measurement method named.
• Which surfaces: cosmetic faces versus all faces.
• The process order: blast, vapor smooth, dye, in that sequence.
• Critical dimensions: features that need to hold tolerance after finishing, so the supplier can compensate or inspect them.
• Color: a reference such as a Pantone code or a physical sample, and whether black is acceptable.
• Quantity: finishing cost per part depends heavily on batch size.

A useful habit is to ask for a measured Ra report on the first article, not only a visual check. That puts both parties on the same number.

Our perspective

Most engineers who contact us about SLS finish ask for “the smoothest option” before they have decided what the part has to do. We think that is backwards. A bracket inside an enclosure needs nothing more than a blast. A housing going to a customer demo needs vapor smoothing and color. Paying for the second finish on the first part is waste, and skipping it on the second is a bad review.

Our approach is to look at the function, the drawing and the quantity first. We are an ISO certified manufacturer with no minimum order quantity, so a single prototype and a small batch go through the same review, and each project has a dedicated project manager. We would rather tell you that a blasted part is enough, or that SLA or CNC is the better process for a cosmetic face, than sell you a finish you don’t need. Tell us the Ra target and which surfaces are critical, and we’ll confirm the route before you commit.

— Nas

Get SLS parts finished the way your drawing says

If you have a part that needs a specific look or roughness, upload your CAD file for an instant quote on our SLS 3D printing service. Note the finish, the critical surfaces and the quantity in the specifications, and your project manager will confirm the details before production. For a broader look at our additive range, see our 3D printing services, and for more on finishing options, read our guide to finishing 3D printed parts.

FAQ

What is the typical surface finish of an SLS part?

As printed, SLS nylon has a matte, grainy texture with an average roughness commonly between Ra 8 and 25 µm, according to an academic review of as-printed polymers. The exact value depends on powder, settings and build orientation. Ask for a measured value if your part has a roughness requirement.

Can you smooth SLS parts, and which method works best?

Yes. Tumbling gives a moderate improvement at low cost, while vapor smoothing gives the largest reduction and also seals the surface. In one Formlabs and AMT study, vapor smoothing lowered roughness by about 72 to 81%. Bead blasting cleans the part and evens the texture but does not smooth it much.

Does vapor smoothing change part dimensions?

It affects only a thin surface layer, but it does change dimensions slightly, and tight-tolerance features can be affected. Flag critical dimensions on the drawing and ask for a first-article inspection after finishing.

Which materials can be vapor smoothed?

Nylon 12 is the standard material for this process, and the AMT PostPro SFX lists it as supported, with TPU and TPE planned. Other materials depend on the equipment and solvent used, so confirm with your supplier before ordering.

Should I paint an SLS part directly?

Paint over a rough surface shows the grain, so prepare the surface first. Vapor smoothing or tumbling before painting gives a far better result than paint alone.

Sources

• Surface roughness of as-printed polymers: a comprehensive review (Springer)
• Guide to SLS Post-Processing Techniques (Formlabs)
• Enhancing the Surface Quality of SLS 3D Printed Parts With Vapor Smoothing (Formlabs, global)
• Enhancing the Surface Quality of SLS 3D Printed Parts With Vapor Smoothing (Formlabs, pricing and equipment)
• Formlabs and AMT vapor smoothing study report
• AMT PostPro SFX Desktop Vapor Smoothing System

Recommended

• 32–500 µin Ra: Bead Blasting vs Sandblasting for Engineers
• What Is Surface Finishing? A Guide for Engineers
• Surface Finishes Guide for Engineers and Designers
• Engineers: Cut Ra 10× with Vapor Polishing Automation and Design Rules