BOB,
I have always took the blade across the windshield , or side glass, It keeps the blade edge sharpe with that rounded edge ,with less jaggies that sand paper will give you sometimes.
But be warned like Bob said , it takes some skill as if you try it with too much pressure or wrong angle, the blade will do some damage.

I have used the file you mentioned , and absolutley hate it , that little tool will tear some paint up ( used it one time and threw it across the shop ), they sell it more for scraping the tops of runs in clear to get flat for rubbing out .....LOL
Its a 1" square fine vixen file on a wood block.

A single edge razor blade can be your friend if you learn to use it well .

3M makes what is called a sanding eraser stone ,1" x 3" x 1" , it comes in several fine grits, soak it in water till saturated then use . You can use the corners just like an rubber eraser for those rock chips too.

Spray