The voids comme from the arachne perimeter generator - since that’s what the slicer uses in the letters, because they’re too small to contain anything but perimeters. You can even see them in the slicer’s rendition:

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There seems to be a way to eliminate then - in the slicer anyway - by increasing the perimeter transition threshold angle from 10° to 40°, but I haven’t tried it yet:

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One-wall looks better anyway, so I’ll try it again, without ironing this time.