How to Fix Blobs and Zits on 3D Prints

Blobs and zits are small bumps or pimples on the outside of a print. Most of them sit in a vertical line, which is the seam. Prusa explains that unless you print in spiral vase mode, each perimeter loop has to start and end somewhere, and that start and end point creates a potentially visible vertical seam, also called zits, layer seams or scars.

That means some seam is unavoidable. Prusa says plainly that with FDM it is not possible to eliminate the seam, but that well-tuned settings should keep it from being very noticeable. The goal is to make the seam small and put it where nobody looks.

Which kind of blob do you have?

1. Move the seam where it hides

In PrusaSlicer the setting is under Print settings, Layers and perimeters, Seam position. Prusa describes the options:

• Nearest places the seam on the nearest edge and, on a model with sharp corners, makes it effectively invisible.
• Aligned lines the seam up through the whole object.
• Random scatters small zits across the surface. Prusa says it makes little sense on models with sharp edges but is viable for cylindrical shapes, and it increases strength.
• Rear puts the seam at the back, useful for objects displayed in a known orientation.

OrcaSlicer offers nearest, aligned, aligned back, back and random, and notes that aligned back, aligned or back usually work best, especially with seam painting.

2. Try a scarf seam or seam gap

Prusa's seam guide also covers scarf joint seams, which overlap extrusions at the start and end of a loop to hide the seam on smooth perimeters. It warns that scarf seams increase print time and are less effective in sharp corners and overhangs. OrcaSlicer's wiki says scarf seams can reduce visible z-seams and recommends printing them at a slow speed (below 100 mm/s).

The seam gap setting controls the distance between the two ends of the loop. OrcaSlicer says a larger gap reduces bulging at the seam, a smaller gap reduces how visible the seam is, and for a well-tuned printer a value between 0 and 15% of the nozzle size is typically optimal.

3. Cut oozing at the start of the seam

OrcaSlicer's wiki explains that a blob at the start of a seam usually happens when filament oozes out of the nozzle during a travel move. It lists these fixes:

1. Calibrate pressure advance. Too low a value leaves excess pressure in the nozzle after the previous extrusion.
2. Make travel speed and acceleration as high as your printer allows, which cuts the time available for ooze.
3. Enable wipe before external perimeters, which moves the de-retraction inside the model.
4. Raise the travel distance threshold so that short moves do not retract (it suggests 2 to 4 mm as a starting point).
5. Enable retract on layer change so each layer starts under the same conditions.

For general retraction tuning and wet-filament checks, read retraction settings explained, fix stringing and how to dry filament.

4. Tune pressure advance (or linear advance)

Klipper's documentation says pressure advance does two useful things: it reduces ooze during non-extrude moves and reduces blobbing during cornering. Prusa describes linear advance as predicting the pressure build-up in the extruder at higher speeds so the firmware extrudes less just before stopping, preventing blobs at sharp corners. Prusa also says its stock presets already include the values, so you only need to tune it for filament that is not covered. Other makers expose the same feature under names such as flow dynamics calibration. Check your printer's documentation.

5. Rule out dry-filament and temperature problems

Hot, wet or poorly dried filament oozes more. Dry the spool, drop the nozzle temperature a few degrees inside the printed range, and check your travel settings again. Related: over-extrusion can also leave bumps along the surface.

Quick test

Print a small cylinder or a Benchy with the same settings after each change, and compare the seam from one angle. Change one setting per print.

The first layer is a frequent starting point for blobs. See how to fix a bad first layer for that.