Zits, blobs and a visible seam on the walls

Symptom

Small zits or bumps appear on the walls, scattered or forming a vertical line (the seam), or plastic builds up at corners and at the start or end of lines.

Likely causes, from most to least likely

1. The seam: every perimeter loop starts and ends somewhere and leaves a mark there. In FDM it can't be removed completely, only hidden.
2. Uncompensated nozzle pressure: when slowing for corners and line ends there is excess plastic (blobs), and too little when starting.
3. Seam set to random, which scatters zits over the whole surface.
4. Over-extrusion (high extrusion multiplier).
5. Wet filament, which leaves lumps on the surface.

Checks to tell which one it is

• If the bumps line up vertically, it is the seam; if they are scattered randomly, check the seam position in the slicer and whether the filament is wet.
• If the bumps are at corners or line ends, it is uncompensated pressure (pressure advance or linear advance).
• Look at the top of a test cube: build-up next to the perimeters means too much material.

Before you start

Before taking anything apart, cleaning or adjusting it, switch the printer off and unplug it, unless a step says otherwise, and do not touch the nozzle, the heater block or the bed until they have cooled down. If in any doubt about electricity, heat or moving parts, stop and ask a professional.

Step-by-step fix

1. Choose where the seam goes

   In the slicer, set the seam to aligned, rear or nearest (on models with corners it hides in them), or paint it by hand where it shows least. Random hides it on cylindrical parts at the cost of scattered zits.

   Check: The seam sits in a barely visible area.

   If that does not fix it: Try a scarf seam.

2. Try a scarf seam

   If your slicer has a scarf seam, enable it on the outer contour to hide it on smooth surfaces.

   Check: The seam mark is reduced.

   If that does not fix it: Calibrate pressure advance or linear advance.

3. Calibrate pressure advance or linear advance

   Calibrate pressure advance (Klipper) or linear advance (Marlin, with M900 K) for your filament using the tower or test pattern from its documentation. It reduces blobs at corners and line ends.

   Check: Sharp corners with no build-up.

   If that does not fix it: Adjust the extrusion multiplier.

4. Adjust the extrusion multiplier

   If there is too much material, lower it in 1-2% steps until the top surface is smooth.

   Check: Fewer bumps on the walls and the top surface.

   If that does not fix it: Dry the filament.

5. Dry the filament

   If you also see strings or bubbles, dry the spool at the temperature and for the time its manufacturer gives.

   Check: The lumps disappear.

   If that does not fix it: If they persist, clean the nozzle and compare your profile with the manufacturer's.

Prevention

• Calibrate pressure advance or linear advance for each filament.
• Keep the seam in a fixed, barely visible position on parts that will be seen from the front.

Still not fixed?

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Printers it applies to

Any FDM (filament) 3D printer, from any brand.

Official sources

This guide summarises the official documentation linked here:

• Seam position · Prusa Research
• Pressure advance · Klipper
• Linear Advance · Marlin
• Drying filament · Prusa Research