second free software i often use is MESHMIXER . it has cool options and fonctions to manipulate STL files => useful for 3D printing or CAM software .
here short videos of some exemples of use that could help you.
convert STL to solid
https://www.youtube.com/watch?v=Cm43dytUfDk
slice a mesh
https://www.youtube.com/watch?v=k-t_ElBXm5o
cut STL profile
https://www.youtube.com/watch?v=hJpLEncCrBM
different manipulations on a STL file for 3D printing
https://www.youtube.com/watch?v=dpbBTcsF7GI