Groove despite 0,01mm gap

For many thin wall objects it makes sense to print reinforcements, stringers etc. as separate objects who melt together with the surrounding shell.

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For some thin wall objects it makes sense to print reinforcements, stringers etc. with a continuous zigzag line, i.e. the extruder turns 90° inwards, moves e.g. 2mm, and returns to continue with the outside shell (saves weight and printing time). Even if drawn with only 0,01mm distance, there is a groove of about half a millimeter on the outside of the object.

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Its not an issue with the Zortrax slicer, its the same with S3D.

Why is this?  With a drawn gap of 0,01mm one would expect that on the outside shell only a seam would be visible.

regards

Andi