Lines will not stay hidden?
What do you mean? Show us an example. Upload an SKP file that illustrates your problem.
As Dave said, upload an example.
Do you have scenes in your model? When you create a scene that has “Hidden geometry” as an attribute, the objects you hide afterwards will become visible again when you return to that scene.
Anssi, yes there are scenes so that could be the problem. I tried to upload but it said file was too big (20 mag). I could send via drop box?
Yes. Upload to Dropbox and share the link.