Say "goodbye" to layer lines! 7 practical techniques to improve the surface smoothness of FDM prints

Are you also frustrated by clearly visible layer lines on your 3D prints? Whether they are aesthetic display models or functional parts with a precise fit, layer lines affect the final quality. The good news is: with a few adjustments, you can significantly improve the print surface. While the nature of FDM printing makes it difficult to eliminate layer lines completely, we can minimize them so they are almost invisible to the naked eye. Here are 7 proven techniques, from basic to advanced, to help you achieve smoother surfaces.

1. Optimizing basic settings: Reducing layer lines at the source

Before trying advanced techniques, ensure your basic print settings are optimized — this is the most important step.
Reducing layer height is key here. By lowering the layer height from the standard 0.2 mm to 0.12 mm or 0.16 mm, vertical layer lines can be significantly reduced. While this increases print time, the surface quality improves visibly.
Linear Advance is another often overlooked but powerful tool. It eliminates blobs caused by sudden acceleration and deceleration of the extruder motor, makes extrusion more consistent, and reduces surface defects. Enable and calibrate it correctly in the Marlin firmware.

2. Calibrate your extruder: Avoid over-extrusion

Over-extrusion is a primary cause of visible layer lines. If more material is extruded than necessary, excess plastic builds up on the sides of the print, creating unsightly vertical lines.
Steps to calibrate the extruder:

• Mark 110 mm of filament from the extruder entry

• Extrude 100 mm of filament through the printer surface

• Measure the actual distance from the mark to the entry

• If the actual length is not 100 mm, adjust the E-steps:
  New E-steps = (Current E-steps × 100) ÷ Actual filament length

3. Controlling print temperature: Find the optimal balance

Print temperature directly affects adhesion between layers and surface quality. Too high a temperature makes the material too fluid and causes "sagging," while too low a temperature impairs layer-to-layer adhesion.
The best way is a temperature tower test: print a tower that has different sections at different temperatures and check which section shows the best surface with the fewest layer lines. Every type of material — and even different colors of the same filament — may require different optimal temperatures.
It is often useful to lower the temperature by 5–10 °C as long as layer adhesion is not compromised.

4. Proper cooling management: Optimal solidification

Fan settings have a major impact on surface quality. Too much cooling can worsen layer adhesion, while too little cooling can lead to deformation.

• For PLA: Turn the fan off completely for the first layer, then increase it gradually up to 100%.

• More cooling may be necessary for small details or overhangs.

• PETG is more sensitive — typically 30–50% fan speed is recommended.

• ABS works best with the fan off, except for small details.

5. Try other infill patterns: Reduce the "ghosting effect"

Certain infill patterns like "Grid" can regularly show through the outer walls and create visible patterns.
Switching to patterns like Concentric, Lightning, or Adaptive Cubic can reduce this effect.
An even more effective method is to increase the wall thickness — e.g., 3–4 walls instead of the standard 2 — which covers the infill better.

6. Advanced techniques: Variable speed & line width optimization

Variable Speed Printing uses different speeds for different areas of the print. For example, slower speed (20–30 mm/s) for outer walls and faster speed for infill and support structures. This improves the quality of the outer walls without significantly increasing the overall time.
Optimizing line width can also help — e.g., set the outer wall line to 90–95% of the nozzle diameter to create particularly densely packed lines and reduce visible gaps.

7. Post-processing: For extremely smooth surfaces

If you are still not satisfied after all the above adjustments, post-processing is the final step.

• Sanding is the most common method: start with coarse sandpaper (approx. 200 grit) and work your way up to finer sandpaper (1000 grit or higher). For PLA, wet sanding prevents the plastic from overheating.

• Chemical polishing results in the smoothest surface: for ABS, acetone vapor polishing works excellently (with proper safety and ventilation). For PLA, an epoxy resin coating can be applied — this smooths the surface and increases strength at the same time.

• Filling and painting: Use a 3D printing filler (e.g., Bondo Spot Putty), fill the layer lines, sand smooth, and then paint. This can produce a surface that is nearly equivalent to injection molding.

Special technique: Non-planar printing

This is the most advanced method for eliminating layer lines. With non-planar printing, the layers are not built horizontally but are aligned along the curvature of the model geometry, which completely avoids horizontal layer lines. However, this technique requires special slicing software and complex settings.

Practical suggestions: Proceed step by step

1. Start by calibrating the extruder

2. Optimize basic settings (layer height, temperature, cooling)

3. Adjust advanced settings (line width, print speed)

4. Try other infill patterns

5. If necessary: carry out post-processing

Do not change too many variables at once. Adjust one setting at a time, print a test piece (e.g., 20 mm cube or Benchy), document the results, and compare the quality. Perfect FDM surfaces are achievable — it just requires patience and systematic optimization.