Among many STL-fixing programs/services these 3 seems to be the most useful in my humble opinion:
1. https://www.netfabb.com/trial-download-confirmation (<--If you use this link you will not have to fill up questionnaire before download. Here you can check how to change from Premium to free, legal, basic version of Netfabb: https://youtu.be/2QRvS9xdNzw made by Thomas Sanladerer)
2. https://service.netfabb.com/login.php - it’s an online service with one big advantage over its full-feature desktop version mentioned above: you don’t have to think about all the settings just upload a file to the cloud and download repaired or not STL file :) (couple iterations of this in case of some STLs sometimes also helps)
3. https://makeprintable.com/ - it is also an online service. I’ve heard positive opinions about it only (no personal experiences)
Source of the first video: fabbaloo.com
