Small bumps on the outer wall surface. They appear at specific spots — almost always at the same position on each layer — and feel like raised dots when you run a finger along the surface.
Identify Where the Blobs Are
Same vertical seam line: Every layer starts and stops at the same X/Y position, leaving a visible vertical ridge of small bumps. This is a seam blob, and it’s the most common type.
Random distribution across the surface: Scattered bumps with no pattern are usually stringing artifacts that got dragged across the surface, or over-extrusion with a partially clogged nozzle.
Only on the outside of corners: Pressure advance (Klipper) or linear advance (Marlin) is not configured, causing the nozzle to over-deposit material coming out of sharp corners.
Fixing Seam Blobs
Move or hide the seam first. Set seam position to “sharpest corner” or “rear of model” in your slicer. This relocates the blob to a corner or hidden face — often good enough that no further tuning is needed.
Enable coasting. Coasting stops extrusion 0.1–0.3mm before the perimeter end, letting residual nozzle pressure fill the gap. Start at 0.1mm and increase by 0.05mm until the seam flattens.
Enable wipe. The nozzle retracts and moves along the last perimeter to smear the seam flat. Use wipe distance of 1–2mm.
Reduce temperature 5°C. Lower temperature reduces ooze at the seam point. If you’re already at the material minimum, don’t go lower — layer adhesion will suffer more than the blobs improve.
Fixing Pressure-Related Blobs
Enable pressure advance (Klipper) or linear advance (Marlin). These firmware features compensate for nozzle pressure lag, which is the primary cause of corner over-extrusion and restart blobs. Start PA value at 0.05 for direct drive, 0.6–0.8 for Bowden, and tune with a PA calibration print.
Fixing Random Surface Bumps
If blobs appear randomly and not at seam lines, check for partial clogging first — run a cold pull. If the nozzle is clear, the issue is usually over-extrusion; reduce flow by 2–3% increments until the surface smooths out.
What Makes It Worse
Extra restart distance set positive (should be negative, typically -0.02 to -0.05mm). Excessive retraction speed — fast retraction creates a vacuum that sucks air into the melt zone and disrupts the next start. Coasting too high (over 0.5mm) creates gaps in the perimeter wall instead of blobs.