There are few surface treatments more visually dramatic than vapour smoothing a fresh FDM print. The visible layer lines disappear in minutes, replaced by a glass smooth, slightly glossy surface that looks like a moulded part. Used appropriately, it is one of the highest impact post processing operations available.
Used carelessly, it damages parts, blurs critical features and produces uneven surfaces that look worse than the as printed finish. The technique repays a bit of understanding.
How vapour smoothing works
Vapour smoothing exposes a printed part to controlled solvent vapour. The vapour condenses on the cooler part surface, partially dissolving and re flowing the polymer at the skin of the part. Once the smoothing time has elapsed and the vapour is removed, the surface re solidifies as a continuous glassy film.
Crucially, only the surface is affected. The part interior is unchanged, so mechanical properties are largely preserved, with some minor gains from surface stress relief. The finish improvement, on the other hand, is dramatic, and on a moulded look part can be the difference between a prototype that reads as printed and one that reads as production.
Which materials are candidates
Vapour smoothing works on polymers whose chemistry is matched to a workshop friendly solvent. ABS smooths beautifully with acetone. ASA also responds to acetone but more slowly. HIPS works with limonene. Some specialty PLA grades smooth with ethyl acetate, though results vary, so test a lot from a single supplier such as OzFDM before committing to a finish on production work.
ABS: acetone vapour, excellent results,
ASA: acetone vapour, slower than ABS,
HIPS: limonene vapour,
Specialty PLA: ethyl acetate, variable,
Not suitable: PETG, PA, PEEK and most engineering grades.
The smoothing process, step by step
Safety first
Acetone vapour is flammable. Use a sealed vessel with controlled venting, keep ignition sources well away, and ensure good general ventilation. Industrial vapour smoothing systems engineer this in; ad hoc workshop setups should not be attempted.
Clean and dry the part. Any surface contamination will read through the finished skin.
Mask critical features. Threads, sealing surfaces and dimensional reference faces should be protected with adhesive vinyl.
Preheat the smoothing vessel to 40 to 60 degrees Celsius for consistent vapour generation.
Introduce the solvent into the controlled vessel and seal it. Never expose vapour to open flame or sparks.
Hold the part in vapour for the planned time: five to fifteen minutes for a light smooth, thirty plus minutes for a deep finish.
Vent the vessel and allow the part to fully off gas before handling, normally several hours.
Inspect under raking light and decide whether a second light pass or paint and airbrushing is the next step.
When we reach for it
We apply vapour smoothing for appearance critical ABS and ASA parts where the moulded look finish is the design intent. Typical use cases are presentation models, appearance prototypes and consumer facing products produced through FDM printing. We do not smooth engineering parts where dimensional precision is the priority, since the same vapour that smooths a surface will quietly soften fine detail.
On larger appearance jobs, we often pair a light vapour pass with a hand sand and paint cycle. The vapour pass removes the bulk of the layer texture, the sand pass evens out any local softening, and the paint pass delivers the final colour and sheen. The result is a part that withstands a close customer inspection without giving away how it was made.