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3D printing troubleshooting

Blobs and zits (pimples on the surface)

Also known as: isolated blobs, dots or warts where each layer starts.

The symptom

Isolated pimples across the walls, often lined up vertically as a 'seam' — spots where the nozzle started or ended each layer and left extra plastic.

Sources for this page: Prusa Knowledge Base · Manufacturer Simplify3D · Technical guide

Causes and fixes, ranked by likelihood

  1. 1

    Layer start with built-up pressure: as the perimeter begins, the nozzle spits the plastic that oozed during travel.

    Fix: Tune retraction (+0.5 mm or +5 mm/s) and enable 'wipe' / 'coast' if your slicer has it. On modern slicers, calibrating pressure advance / linear advance kills the start blob.

    Source: Simplify3D — Print Quality Guide: Blobs and Zits ↗
  2. 2

    Seam in the worst spot: every layer start lands on the same vertical of a visible wall.

    Fix: Move the seam: 'random' hides it by scattering, 'nearest/sharpest corner' tucks it into edges. For figurines, paint the seam onto the back side.

    Source: Simplify3D — Print Quality Guide: Blobs and Zits ↗
  3. 3

    High temperature or wet filament: plastic drools on its own, leaving warts away from the seam.

    Fix: Drop 5-10 °C and dry the filament. If the pimples come with fine hairs, start there (see the stringing page).

    Source: Prusa Knowledge Base — Print quality issues ↗

hotend tools that help here

Printer comparator → Stock pressure advance and flow calibration make the seam difference: check which models ship them in the comparator.

Related

FAQ

Can I remove pimples from a finished print?
Yes: flush-cut with a blade when cold and, if the material allows, a quick pass of hot air (heat gun at a distance) or fine sanding. But calibrate the seam for next time: shaving a hundred pimples gets old.
What exactly is 'pressure advance'?
A firmware feature that anticipates hotend pressure: it pushes less right before stopping and more right before accelerating. Result: spit-free layer starts and bulge-free corners. Klipper, Marlin (linear advance) and Bambu/Prusa firmwares ship it.

Sources & references

This page is cross-checked against public manufacturer documentation and reference technical guides:

Still stuck? Back to the quick diagnosis or check which materials your printer can handle.