Hi again
I also use Sketchup and I think I’m understanding your problem.
You really cannot fix incorrect stl files in a slicer.
You can only fix problems like holes in the stl file.
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Your source file from Sketchup, I believe is your problem.
Sketchup does not make true circles.
Sketchup makes polygons that have the circle transcribed about the outside of the polygon.
You can change the polygon count of circles in Sketchup.
I’m not sure if this works in all versions of Sketchup, but it does work in SketchupPro 2014 thru 2016
When you select the circle tool, take a look at the lower right hand side of your screen.
There, It will say how many facets the circle will be built from.
The default is 24 facets.
You can change that.
If I need a circle that looks round when the diameters are greater than about 2 inches, I usually us a 60 facet count, Sometimes 80.
That definitely improves the 3D printed output.
…
When I export a model I’ve designed in Sketchup to STL, I always reload the STL back into Sketchup and check it with
Solid Inspector plug in to be sure there are no reversed faces or holes in the STL.
I hope this helps you in some manner
gg