Offset and problems with GL_SMOOTH


i have problems in painting a grid together with a surface. At first I paint the grid and then the surface in glShadeModel(GL_SMOOTH).
Because I had the problem, that sometimes the lines were visible and sometimes covered by the surface I tried out the offset command


Now the lines are visible but the ShadeModel seemes to have been changed to GL_FLAT.

Has anyone an idea what’s wrong.