Blobs and zits on 3D prints: how to get rid of them

Blobs and zits are small bumps on the surface of a 3D print, left wherever the nozzle starts, stops or pauses on an outer wall. Where they appear tells you the cause: a vertical line is the seam, bumps on every corner mean pressure advance needs tuning, and random zits usually come from retraction and ooze.

Identify the bump first

Look closely at where the bumps sit on the print. Each pattern has a different fix.

Z seam position: Aligned, Rear, Nearest or Random

Every wall loop has to start and finish somewhere. That joint leaves a tiny bump or dip, and stacking them up the part creates the Z seam. You can't remove it entirely, but you can choose where it goes.

• Aligned or Rear/Back gives one tidy line. Best for parts with a back you won't see.
• Nearest or Sharpest Corner tucks the seam into corners, where it's almost invisible on boxy parts.
• Random spreads the joints over the whole surface. There's no line, but every wall gets a light texture of zits, so it rarely looks better.
• Seam painting lets you draw exactly where the seam goes, for example along a hidden edge.

On boxy parts, put the seam in a corner. On round parts, put it at the back or use a scarf seam.

Don't pick Random to hide a seam on a smooth cylinder; you'll trade one line for hundreds of zits.

Scarf seams

A scarf seam ramps the end of each loop up and the start down over a short distance, so the two ends overlap gradually instead of meeting at one point. It works best on smooth, curved walls. It's available in OrcaSlicer 2.0 and later, Bambu Studio 1.9 and later, PrusaSlicer 2.9 and later, and Cura 5.9 and later (as an experimental setting). Settings and names differ, so start from your slicer's defaults and adjust gradually.

Once pressure advance is tuned, a slightly larger seam gap (the small space left between the two ends of a loop) reduces bulging at the joint. OrcaSlicer's seam guide also notes that the inner/outer/inner wall order typically gives the cleanest seams, followed by inner/outer.

Pressure advance and linear advance

Molten plastic in the nozzle acts like a spring. When the toolhead speeds up, pressure lags, so line starts come out thin. When it slows for a corner, pressure is still high, so plastic bulges out. Pressure advance (Klipper, Prusa, Bambu, OrcaSlicer) and Linear Advance (Marlin) push a little extra filament during acceleration and pull back during deceleration to keep the flow matched to the speed.

• Bambu Lab: called Flow Dynamics Calibration. Many models can run it automatically before a print; you can also run it manually from Bambu Studio's Calibration tab and save the result per filament.
• Prusa: the MK4/S, XL, MINI and CORE One use Pressure Advance set with M572 in the PrusaSlicer filament profile's custom G-code. The MK3 series uses Linear Advance with M900. Prusa's own filament profiles already include tuned values.
• OrcaSlicer: Calibration → Pressure advance prints a tower, line or pattern test.
• Klipper: print a tuning tower and read off the height where corners look best.
• Marlin: enable LIN_ADVANCE in the firmware, print Marlin's Linear Advance calibration pattern, then set the K-factor with M900 K.

Tuning pressure advance on Klipper

1. Calibrate rotation_distance and temperature first. Pressure advance tuning assumes extrusion is already correct.
2. Disable scarf seams and dynamic acceleration control in the slicer for the test print.
3. Run the commands below, then print Klipper's square tower test object. Use FACTOR=.020 instead of .005 for long Bowden setups.
4. Find the height with the sharpest corners and calculate pressure_advance = START + height × FACTOR. Save it in [extruder].

SET_VELOCITY_LIMIT SQUARE_CORNER_VELOCITY=1 ACCEL=500
TUNING_TOWER COMMAND=SET_PRESSURE_ADVANCE PARAMETER=ADVANCE START=0 FACTOR=.005

[extruder]
pressure_advance: 0.040   # example: best corners at 8 mm, 0 + 8 x 0.005

Values vary by extruder and filament. Klipper's docs quote a typical range of about 0.050 to 1.000, with the high end usually only on Bowden extruders; many direct drive setups land at the low end or just below it.

Setting linear advance on Marlin

Marlin recommends adding the K-factor to the slicer's start G-code so you can change it per filament:

M900 K0.08   ; Linear Advance K-factor from the test pattern (example)

Retraction, wipe and coasting

Random zits usually appear where the nozzle retracts, travels and restarts on an outer wall. A tiny bit of ooze or under-primed start leaves a mark each time.

Retraction length mostly drives stringing, so if you're also seeing hairs between parts, tune it with the stringing and oozing guide. Scars and drag marks on top surfaces are a slightly different problem: try Z-hop only over printed parts, and check you aren't over-extruding, which leaves ridges for the nozzle to catch.

The Benchy hull line

The Benchy hull line is a single horizontal line around the hull at roughly the height of the deck. Prusa's research points to a sudden change in layer time where sparse infill turns into solid deck layers, and again where the deck ends. The walls cool and shrink differently on those layers, which leaves a visible step.

It's not a hardware fault, so tightening belts or replacing Z rods won't help. Its severity depends on the filament, cooling and room temperature. Prusa reduced it in its sample G-code using a modifier mesh in the shape of the deck to separate the deck from the hull walls. For everyday prints, accept that it's a known quirk of this test model rather than a sign your printer is broken.

If you see uneven lines all the way up the part instead, that's a different issue; start with the 3D printer calibration guide.

Frequently asked questions

Related articles

Tools for thisAll tools

G-code ViewerInspect a sliced file layer by layer: paths, travels, speeds and filament use.

Flow Rate CalculatorDial in the extrusion multiplier from a measured single-wall cube.

Watch it doneVideo library

1:05 Learn 3D printing in 7 days, day 7, part 2: the right-click menuTutorials 1:01 Learn 3D printing in 7 days, day 5, part 2: support, seam and color paintingTutorials 1:05 Learn 3D printing in 7 days, day 5: slicer tools to place your modelTutorials

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