As nophead says, a lot of STL's on  Thingiverse and other places are defective, and not just in their geometry. If there's an STL I can't be bothered to recreate with OpenSCAD, I use an online converter to create an object which downloads an oject list of points and can then be properly rendered when you hit F6, this means you can create the object and modify it just like you would any other OpenSCAD object. Here's the source, https://github.com/raviriley/STL-to-OpenSCAD-Converter And here's the online site. https://raviriley.github.io/STL-to-OpenSCAD-Converter/ The results are very long lists of point coordinates - which are best <include> as a separate file, it gives my rapidly ageing laptop a headache and there's no point whatsoever trying to edit the points. N.B. I did find a way of self-hosting the entire application on a webserver inside my network.. HTH. Roger