This is a very simple and powerful service offered by Microsoft and powered by netfabb. Using a simple 3 step process, the tool will automatically fix many common problems with 3d models. The tool supports STL, OBJ, 3MF and VRML files.
Step 1: Point Your Browser of Choice to the Model Repair Service Website
As you can see in the screenshot, the URL is https://modelrepair.azurewebsites.net.
Step 2: Sign in to the Model Repair Service
You'll need a Microsoft account to access the site. Click the Sign In button, a pop-up window will allow you to sign in using your Microsoft account. If you don't get the login window, please check any pop-up blocker you may have and allow the pop-up window.
Step 3: Upload Your File
You'll see a very simple interface after logging in. Simply click the upload button and select the file you would like to repair. The file will upload and automatically queue for repair. When the file is repaired, move on to the next step.
Step 4: Download Your Newly Repaired File!
Click the now lit up DOWNLOAD button to download your newly repaired file. Very simple and very powerful. I've had great luck with the tool and tend to run all my STLs through it before printing just to ensure I will have a good file. Remember the old adage, "Garbage in. Garbage out".