Helllo!

Thank you BurrMan for tutorial! It works for me now)

Thanks Michael, it's more clear now for me. But still needs more time and practice to get it this type of workflow, after poly modeling

So, I fixed that places on my model. But after trying to apply same method to another parts, I faced with another issue: that new surface doesn't weld with geometry.

On screenshot - I marked hole in my mesh, and after trying to fix that on the right side ( and it didn't work) and after reopen file I get this wierd issue.
If I'm selecting and delete that surface, I get that (second screen shot). And Its hard to fix - some edges couldn't be deleted or separated.
I attached model file.
Thank you very much guys for help.