I think the problem is how you created the lines.  I would venture a guess
that you created the lines by entering a line (dashes or underscores) into
the cell.  Yes, you will overwrite that line when you enter something else
into that cell.
   Instead, create the lines by using borders (Format - Cells - Borders).
You can select the type of line you want and how thick a line.  That line is
then a border and is not a cell entry.  Post back if I'm off track with your
problem.  HTH   Otto