The following programs work with 3D models, providing functionality that SketchUp lacks:
Netfabb Basic at http://www.netfabb.com/basic.php is a free program for manipulating and fixing errors in STL files.
Netfabb Cloud at https://netfabb.azurewebsites.net/ is a powerful free STL repairing hosted online. Use this to clean up models and fix errors such as missing faces, and internal geometry.
Meshlab at http://meshlab.sourceforge.net/ is used to convert many files to import into SketchUp or convert models exported from SketchUp.