If you're going to tackle any filler work you'll need the right tools for the job. Here we have three flexible sanding blocks, the longer two of which have three removable rods running their length which can be removed to make the blocks more flexible for sanding curved areas, as we'll see. The longboard sander at the bottom of the picture is an old rigid version. At top center is a DA (dual action) orbital sander. Electric versions of these are available and may work for the hobbyist.