Separating a model into pieces

[gnat]

Background: I just found out about Wings3D and barely know what I'm doing in 3D modeling (TinkerCAD is my go to option...), so I apologize ahead of time for lack of understanding. My primary usage for 3D models is for printing.

I have a model that is a single object in an STL and I would like to separate parts into sub objects so that they can be printed individually. I've played with MeshMixer to attempt this after reading and watching tutorials, but so far I have had no success (the results are non-printable objects).

Playing with Wings3D it seems to have a much nicer interface that MM in regards to identifying the parts I want to break out, but even though it feels like W3D has the feature I'm looking for I can't seem to figure it out.

Is this something that can actually be done? If so, what is the proper workflow?