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