Smooth surfaces: layer height, ironing, seams, sanding, primer, vapor smoothing

Layer lines are the signature of FDM. For some parts that is fine, but a display piece, a lid people touch every day, or anything that will be painted needs better. Smooth surfaces come from two stages: in the slicer before printing, which is free but costs print time, and after printing, which costs hand work and care with chemicals. This lesson goes from cheap to expensive and ends with vapor smoothing, where safety comes before results.

Thinner layers: the first thing to try

The line you see is the layer height. Lower it and the lines shrink, within two limits.

• Upper: keep layer height below 80% of the nozzle diameter. For a 0.4 mm nozzle that is about 0.32 mm.
• Lower: below 0.10 mm the gain is small while print time soars. 0.07 or 0.05 mm looks only slightly better.

Example: a part 30 mm tall.

Layer height	Layers	Approx. time (vs 0.2)	Top layers for ~0.9 mm thickness
0.28 mm	107	0.7×	3–4
0.20 mm	150	1×	4–5
0.12 mm	250	~1.7×	7–8
0.08 mm	375	~2.5×	11–12

Time is not exactly linear, but close. The sweet spot for display parts is usually 0.10–0.12 mm. Layer height only helps the Z axis; horizontal detail depends on the nozzle, and thinner layers need more top/bottom layers for the same thickness.

Variable layer height uses thin layers only on curves and slopes where the steps show: a surface close to 0.1 in a time close to 0.2.

Ironing: pressing the top

Ironing sends the nozzle back over the top surface on the same layer, extruding a very thin line to fill the grooves. Flat tops such as nameplates or lids come out glossy.

• Ironing type: All top surfaces or Topmost surface only; All solid surfaces is experimental, for 100% infill parts.
• Flow rate: a percentage of normal flow, kept low. Too little leaves grooves; too much drags plastic into a ridge at the edges.
• Spacing: the distance between ironing lines, smaller than the nozzle diameter. Smaller is smoother and slower.
• Speed: on a different page (PrusaSlicer: Print Settings, Speed, Ironing). Slower works better.

The trap: filament moves through the hotend very slowly during ironing, so heat climbs and can cause a clog (heat creep). PLA is most at risk, especially in hot weather. Ironing also needs well-tuned flow; if flow is off, no ironing makes it flat.

Seams: hiding the join

Every layer’s outer loop has a start and an end. Keep them in the same place and you get a vertical line; scatter them and you get small dots all over.

Option	What it does	Suits
Nearest	Finds a nearby concave corner and hides the seam there	Parts with sharp corners
Aligned	Picks like Nearest but matches the previous layer: one straight vertical line	Cornered parts, tidy seam
Rear	Like Aligned, pushed toward the back of the bed	Display pieces that face one way
Random	A different spot every layer, scattered as small dots	Cylinders with no corners

Seam painting lets you paint where the seam must go (enforcer) or must not (blocker). Scarf joints overlap the line ends at an angle, softening the bump on smooth curves at the cost of time. A blobby seam is usually too much flow: trim the extrusion multiplier slightly and tune pressure/linear advance.

Fuzzy skin: not smooth, but hidden

If the side walls only need to be free of visible lines, fuzzy skin jitters the outer loop in and out to make a texture like moulded plastic grain. Fuzzy skin thickness (around 0.3 mm is common) is the maximum a point can move; Point distance (around 0.8 mm) is the spacing between points. Dimensions change by the thickness you set, so keep it off mating surfaces.

Sanding and primer: works on everything

The classic path for painted parts. Print at 0.12–0.16 mm with at least 3 walls, because sanding removes material.

1. Cut away supports and clean up with flush cutters and a knife.
2. Sandpaper at 220 or 320 to take down the lines, then 400 and 600. Sand in circles, not along the layers, and wet-sand the fine grits to cut dust and stop the paper clogging.
3. PLA softens at about 55–60 °C. Sand it fast, or with a power tool, and it gums up instead of smoothing. Go gently and let it cool.
4. Wash off the dust, dry, then spray filler primer (a primer with body that fills grooves, sold at car parts shops) in 2–3 thin coats, drying and sanding at 400 or finer between coats. Fill deep grooves with spot putty in thin layers.
5. Paint in 2–4 thin coats rather than one thick one, then 1–2 coats of clear.

Every spray goes on in a ventilated space with an organic vapor respirator and nitrile gloves. Sanding dust is not for breathing either.

Vapor smoothing ABS/ASA: safety first

ABS and ASA dissolve in acetone. Its vapor swells the surface, which flows until the layer lines vanish, leaving a gloss like moulded plastic. But acetone is highly flammable, and this lesson describes no method that uses heat.

Acetone’s flash point is −20 °C, so it ignites at any room temperature. The vapor is about twice as dense as air; it flows downhill toward a flame or a floor heater and can flash back to the container.

Non-negotiable before you start

• No flame, spark or heater in the same room. No warming acetone on the print bed, no hair dryer.
• Work outdoors, or beside a window with a fan blowing out of the room, not toward you.
• Chemical splash goggles, gloves (nitrile for brief contact of a few minutes; butyl rubber for longer handling) and an organic vapor respirator.
• A glass container, never plastic, and never airtight: a loose lid, or one with small holes, so pressure cannot build.
• Never leave it unattended.

The cold vapor method

1. Line the bottom of a glass container with paper towel soaked in 100% acetone (not nail polish remover), and stick more around the inside walls.
2. Stand the part on a small platform (aluminium foil or a glass lid). It must not touch the paper or any liquid.
3. Lid on loosely, at 18–25 °C, and check every 15–20 minutes. At about 15 minutes the surface gains a sheen, at 30 the lines fade, at 45–60 it is fully glossy. Some ASA is done in 15–20 minutes; some ABS takes an hour, and a few strongly pigmented filaments still show lines.
4. Lift it out with gloves, do not touch the surface, and leave it in a ventilated place. It feels dry in 1–2 hours but fully hardens in 4–8 hours, up to a day. Paint before that and the paint bubbles.

What you give up: small text, sharp edges and fine detail round off; thin or flat areas can warp; dimensions change by 0.1–0.3 mm and holes close up. For mating parts (see the dimensional accuracy lesson), mask that surface or add clearance in the model. PLA, PETG, nylon and TPU do not respond, and the solvents that work on PLA/PETG are restricted and far more dangerous than acetone. Do not go looking for them.

Epoxy or resin coating

For PLA/PETG, a thin brushed coat of clear epoxy or UV resin self-levels into the grooves and cures by time or lamp. Gloss and a harder skin, but dimensions grow by the coat and corners round off. Gloves and ventilation again: uncured resin irritates skin and is a sensitiser.

Comparing the methods

Method	Works on	Finish	Dimension change	Risk
Smaller layer height	Every material, every surface	Better all over	None	Print time
Ironing	Flat tops	Glossy, flat	None	Heat creep (PLA)
Seam/scarf	Vertical walls	Removes the vertical line	None	Time
Fuzzy skin	Vertical walls	Textured, lines hidden	+/− the thickness set	Not for mating parts
Sanding + filler primer	Every material	Smooth, ready for paint	−0.1 to −0.3 from sanding	Dust, spray fumes
Vapor smoothing	ABS, ASA (and HIPS)	Moulded gloss	0.1–0.3 mm	Fire, vapor, lost detail
Epoxy/resin coat	PLA, PETG	Glossy	Grows by the coat	Skin irritation, fumes

Common mistakes

• Dropping the whole part to 0.05 mm layers. Four times the print time for barely better than 0.1. Use variable layer height.
• Ironing before tuning flow, getting grooves or a ridge at the edges, and blaming the feature.
• One thick coat of primer to fill everything at once: runs and lost detail.
• Warming acetone on the print bed or a stove. Faster, and the easiest way to start a workbench fire.
• Painting a vapor-smoothed part after a few hours. Acetone still in the plastic pushes the paint into bubbles.

The cheapest smooth surface needs no extra work: put the pretty face on top or vertical, set 0.12 mm layers and the seam at the back.

Sources

• Layers and perimeters (layer height vs nozzle, top layers) — Prusa Knowledge Base
• Ironing — Prusa Knowledge Base
• Seam position — Prusa Knowledge Base
• Improve your 3D prints with chemical smoothing — Prusa blog
• Acetone vapor smoothing for ABS: safety guide — QIDI print lab