Blobs, zits and scars — how to clean up surfaces
Blobs are small bumps where extrusion stopped and started. Zits are smaller, randomly placed surface bumps. Scars are gouges where the nozzle dragged across a printed surface. All three are cosmetic, all three are fixable with three settings: seam position, pressure advance, and retraction wipe.
Set seam position to "Aligned" or "Rear" (hides the seam in one corner instead of scattering it). Enable retraction wipe (drags the nozzle on retract so ooze stays on the wall, not on the next start). Tune pressure advance (Klipper/Bambu) or linear advance (Marlin) — bad PA leaves a blob at every corner.
The Z seam: one vertical scar per layer
Every layer has to start and stop somewhere on each perimeter. That start/stop point becomes the visible Z-seam — a vertical line of tiny bumps running up the print. Slicer options for seam placement:
| Setting | Result | Use when |
|---|---|---|
| Aligned | One vertical seam in a consistent spot | Default for most prints |
| Rear | Seam on back face only | Prints have a clear "front" |
| Random | Spread the seam over many tiny dots | Round prints where any line is jarring |
| Nearest / Sharpest | Hides seam in concave corners | Designed parts with corners |
| Painted (Bambu, OrcaSlicer) | Manual seam path | Models with a "designed" back side |
Pressure advance / linear advance
When a perimeter ends, melt-zone pressure keeps pushing plastic out for a moment after the extruder stops. That extra plastic lands as a small zit at the end of every line. Pressure advance (PA) and linear advance (LA) tell the firmware to back off the extruder slightly before a deceleration so the pressure equalizes by the time the line ends.
- Bambu: auto-calibrated. Visible as "Dynamic flow calibration" before some prints. Re-run from Device → Cal → Flow Dynamics.
- Klipper: run
TUNING_TOWER COMMAND=SET_PRESSURE_ADVANCE PARAMETER=ADVANCE START=0 FACTOR=.005. Pick the layer with sharpest corners. - Prusa Buddy (MK4): on by default; tune via the Connect dashboard if needed.
- Marlin (linear advance): use Marlin's K-factor tower test and set with
M900 K<value>.
Typical values: 0.02–0.06 for direct drive PLA; 0.4–0.8 for Bowden Marlin.
Retraction wipe and coast
- Wipe on retract (or "outer wall wipe distance") drags the nozzle 1–2 mm along the just-printed wall as it retracts. Ooze deposits on the wall instead of in the air.
- Coast at end stops extrusion 0.1–0.4 mm before the line ends so any residual pressure finishes the line for you. Especially effective for PETG.
- Avoid crossing perimeters reroutes travel moves through infill so any drop doesn't scar a visible wall.
The Benchy hull line
The "Benchy hull line" is a horizontal scar across the side of the boat at a specific layer height. It's caused by the slicer changing speed mid-layer when transitioning from solid bottom to perimeter-only walls. The melt zone is at a steady-state pressure at one speed; switching speeds disrupts that.
Fixes:
- Enable "Smooth speed change" or "Speed transition" in OrcaSlicer / Bambu Studio.
- Lower outer wall speed so the layer where the bottom ends doesn't see a big speed change.
- Tune pressure advance — a well-tuned PA value removes most of the hull line by itself.
- Print Bambu-Lab-bundled Benchy files in 1 colour from external slicers to avoid the multi-tool transition that worsens the line.
Other zit causes
- Wet filament: moisture bubbles burst at the nozzle, leaving micro-zits. Dry the filament.
- Over-extrusion: too much plastic has to go somewhere — usually as bumps on the surface. Lower flow rate.
- Travel without Z-hop: the nozzle scrapes wall tops; melt sticks to the tip and gets deposited later. Enable 0.2–0.4 mm Z-hop.
- Filament rubbing the spool guide: shavings drop into the extruder and randomly extrude. Inspect path; clean extruder.
Related articles
Sources & further reading
- Prusa Knowledge Base — The Benchy hull line
- Klipper Docs — Pressure Advance
- Bambu Lab Wiki — Flow Dynamics Calibration