Fixing broken .stl model

[knightersunit345]

Hello, I come from Fusion 360 and am new to Meshmixer

So have to print this head but the file is messed up(Screenshot 1). So I import into Meshmixer but the error from TinkerCad are not visible(Screenshot 2).  I use the make solid feature but then I get all these new mess ups on the model(Screenshot 3). After I have reduced the model and brought it back into Tinkercad, the original errors were fixed but now I have all these new errors(Screenshot 4). 

Why do the errors from Screenshot 1 show up in Tinkercad and PrusaSlicer, but not Meshmixer.

Why is the model making these new errors after I have used Make Solid.

How do I fix these new errors and make the head look normal?

Any help is appreciated, whether it is directly or a link to a video.

    

[MagWeb · Accepted solution]

Unfortunately there are tons of errors in your source and it would need a long essay of how to fix them.

Run ANALYSIS/Inspector to see them. But on these hard to repair issues Inspector's repair algorithms will fail. Also MakeSolid is confused by the different directions of inconsistent regions. So one need to do this manually.

Main problem is an interior vertical plane cutting through the head which results in many non manifold issues.

My basic strategy on such a bad file is to separate parts (partial objects like the eyeballs or surface regions) that look ok and to join them to one later again. This isolates bad stuff and one can use different repair strategies (Remeshing, Bridging, EraseAndFill...) on easier parts. In SELECT>Select the region to separate and run Edit/Separate...

Good news is: I fixed the errors for you. See attachment