Change your sandpaper to 80-grit and repeat the process ... and overlap your passes by about 1/3 of the sanding disc. Don't skip a grit: You'll have great results. Also, don't push down on ...

In most cases, 80-grit paper is a good starting point for ... of the putty knife with a utility knife. Adhesive-backed sandpaper discs are readily available at home centers and hardware stores.