3D Print Blobs and Zits: How to Fix the Small Bumps on Your Walls

Why every extrusion start and stop leaves a mark

A 3D printer doesn't lay down plastic in one continuous bead. Every time a perimeter loop ends, the extruder stops, the nozzle travels, and extrusion starts again somewhere else. According to the Simplify3D troubleshooting guide, most extruders produce a very uniform extrusion while they are running. The variation shows up each time the extruder is turned off and on again. That brief surge leaves a tiny bump of extra plastic on the wall. Makers call it a blob or a zit.

The Prusa Knowledge Base describes the same thing from the seam angle. Unless you print in spiral vase mode, every perimeter loop has to start and end somewhere. That start and end point creates a potentially visible vertical seam, also called zits, layer seams, or scars. On a cylindrical object with no sharp corners, the seam is obvious. On a boxy part with crisp edges, it can nearly vanish.

Set your expectations before you start tuning. Prusa is blunt about it: with FDM printing, the seam cannot be eliminated and will always be somewhat visible. The goal of this guide is to make it unobtrusive, not invisible. Isolated blobs, on the other hand, respond well to a few targeted slicer changes, and that's where the real wins are.

Watch the nozzle: does the bump form at the start or the end?

Before you change anything in the slicer, watch the printer work. Simplify3D says the best way to diagnose surface defects is to watch closely as each perimeter is printed. Ask one simple question. Does the bump appear the moment the extruder starts the perimeter? Or does it only appear later, when the loop is complete and the extruder is coming to a stop?

That one observation decides which setting you touch. A blob at the start points to over-priming: the extruder is pushing too much plastic back into the nozzle when it restarts. A blob at the end points to pressure still trapped in the nozzle: plastic keeps flowing as the toolhead slows down. The two problems have different fixes, which the next two sections cover one at a time. Don't mix them up.

Print a plain test object rather than your real model while you diagnose. A simple cylinder or cube makes the seam and its bumps easy to spot without the distraction of geometry. If what you see is thin hairs stretching between two parts of the print rather than bumps on the wall, that's stringing, and the cure starts with your retraction settings rather than the seam.

Blobs at the start of a loop: dial back the restart distance

When the extruder retracts filament before a travel move, it has to push that filament back before printing resumes. If it pushes too much, the surplus lands at the start of the perimeter. According to Simplify3D, the setting that controls this is called Extra Restart Distance. It sets the difference between the retraction distance used when stopping and the priming distance used when restarting.

Simplify3D gives a concrete example. With a retraction distance of 1.0 mm and an extra restart distance of -0.2 mm, note the negative sign, the extruder still retracts 1.0 mm every time it stops. When it restarts, though, it only pushes 0.8 mm of plastic back into the nozzle. That small deficit accounts for material that oozed out in the meantime. The guide recommends adjusting this value until the defect no longer appears at the beginning of the loop.

Other slicers expose similar controls under names tied to retraction or priming, so look for the equivalent in your software. Get your base retraction distance and speed right for your extruder type first, then fine-tune the restart amount. Change one parameter at a time and reprint the same test object so you can compare walls side by side.

Blobs at the end of a loop: coasting, wipe, and fewer retractions

If the bump appears as the nozzle finishes the perimeter, the culprit is pressure built up inside the hotend. Simplify3D's answer is coasting. It turns the extruder off a short distance before the end of the perimeter so that pressure bleeds off into the last stretch of the loop. The guide states that a coasting distance between 0.2 and 0.5 mm is typically enough to have a noticeable impact. Raise it in small steps.

Bowden setups build more pressure because of the long tube between the drive gear and the nozzle, so a standing retraction tends to leave a blob. Simplify3D offers a non-stationary retraction for this case. Enable Wipe Nozzle with a wipe distance of 5 mm as a starting point, then turn on the option to perform the retraction during the wipe movement. The nozzle keeps moving while it retracts, so there's no stationary moment for a blob to form.

Sometimes the best retraction is the one you skip. Simplify3D recommends the option to retract only when crossing open spaces: any ooze then lands inside the model where nobody sees it. Pair it with the option that routes travel moves around the outline so the nozzle crosses perimeters less often, which means fewer retractions and fewer chances for a zit.

Picking a seam position that hides the start point

Once isolated blobs are under control, the seam itself is what's left. According to the Prusa Knowledge Base, PrusaSlicer offers several seam position strategies under Layers and perimeters, with some only visible in Expert mode. Nearest places the seam on the closest edge of the current layer. On a model with sharp corners it becomes effectively invisible, because the slicer first looks for a concave, non-overhanging vertex, then a convex one, before settling for anything else.

Aligned uses the same candidate logic but picks the point nearest the start of the previous layer. The result is one straight vertical seam running up the part, which is easy to orient away from view. Rear works like Aligned but pushes the seam toward the back of the bed. Prusa recommends it for objects that will be displayed in a specific orientation.

Random picks a different point on every layer. Prusa notes that the seam becomes less noticeable at the cost of a less smooth surface, with small zits scattered across the walls, while overall strength goes up. It makes little sense on parts with corners, where Nearest or Aligned do a better job. On a smooth cylinder with no edges, Random is a reasonable choice. Most other slicers offer comparable seam options under similar names.

Scarf seams, flow, and Linear Advance when the seam still shows

Prusa also documents the scarf joint. Instead of a hard stop, the start and end of the loop overlap along a ramp that climbs gradually to full layer height. Prusa says this reduces visible z seams, especially on curved surfaces, and improves overall part strength. The trade-off is longer print time, and it's less effective in sharp corners and overhangs. By default the scarf is applied only to smooth perimeters, and the height change is divided into 1 mm segments.

If the seam still stands out, look at flow. Prusa points to two settings that directly affect seam visibility: the extrusion multiplier and Linear Advance. Stock Prusa profiles already have them tuned. On a custom profile, try lowering the extrusion multiplier slightly and calibrating Linear Advance. Our guide to 3D printer calibration walks through that process. A print that suffers from general over-extrusion will show fat seams no matter where you put them.

The last resort is vase mode. Prusa reminds you that it avoids the seam entirely, but limits you to a single perimeter and no infill. That suits decorative hollow objects, not parts that need to carry a load.

Sources and limits

• Simplify3D — Blobs and Zits
• Prusa Knowledge Base — Seam position

We organize manufacturer and community documentation; we have not measured these products ourselves. The manual for your exact model takes precedence over general guidance.