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.
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:
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.
Typical values: 0.02–0.06 for direct drive PLA; 0.4–0.8 for Bowden Marlin.
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.