TRIDYsign
Language: ES

Blobs and zits: bumps on the surface

Severity: MediumAffects: Filament (FDM)Updated July 27, 2026

Symptom

Small bumps or zits appear on the walls, almost always aligned vertically at the seam where each layer starts and ends. In severe cases a mass of plastic builds up around the nozzle and can ruin the whole print.

Likely causes

In order: the most frequent cause comes first.

  1. 01Residual pressure in the nozzle when starting and ending each perimeter
  2. 02Uncalibrated retraction and pressure advance
  3. 03Temperature too high, making the material runnier
  4. 04Damp filament producing bubbles and irregular bursts
  5. 05Plastic residue stuck to the nozzle that drops onto the part

How to fix it

Apply one fix at a time and reprint the same test part. Changing three settings at once tells you nothing about which one worked.

  1. 01

    Calibrate pressure advance

    Medium

    Pressure advance (called linear advance on some machines) compensates for the pressure that builds inside the nozzle during acceleration and deceleration. OrcaSlicer has a built-in test to calibrate it, and it is the fix that cleans up the vertical seam most effectively.

  2. 02

    Hide the seam and lower the temperature

    Easy

    In the slicer, set seam position to 'random' or 'nearest corner' so the zits do not line up in a visible stripe. Also drop 5 to 10 °C: less temperature means less ooze at the start point.

  3. 03

    Dry the filament and wipe the nozzle

    Easy

    Irregular zits scattered over the whole surface, rather than lined up on the seam, are almost always moisture. Dry the spool and wipe the nozzle while hot with a cotton cloth or a brass brush before each print.

How to prevent it

  • —Calibrate pressure advance once per material and save it in the profile
  • —Wipe the nozzle hot before each important print
  • —Store filament with desiccant
  • —Use a random seam on parts where the vertical line would show

Sources

  1. [1]Print Quality Troubleshooting | Prusa Knowledge BasePrusa Research · Manufacturer · accessed July 27, 2026

Related problems

MediumFDM

Stringing: threads and cobwebs between parts

Fine plastic strands are left between two parts of the model, or between two objects on the same plate. Prusa calls it 'hairy prints'. In severe cases the part is covered in a web you have to remove with a knife or heat gun.

MediumFDM

Over-extrusion: more material comes out than needed

Walls come out bulging and rough, top layers show piled-up material, holes print smaller than designed and parts that should fit will not go together. On print-in-place assemblies, moving parts end up fused.

LowFDM

Ringing or ghosting: echoes and ripples on the walls

After each corner or sharp change of direction, repeating ripples appear along the wall and fade out, like an echo of the previous feature. It shows up most on flat surfaces and around lettering or holes.