Sanding my scarf joint

One of the purposes of a scarf joint is to maintain the plywood's thickness.  When sanding down the first of my scarf joints I came up with an idea for a "leveling" sanding block.

 A found a good flat piece of red oak scrap in my shop and checked it's flatness with a metal straight edge.  I then affixed sand paper to the center of it, using a 6 inch peel-n-stick sand paper intended for a random orbital sander.  I don't know why I had this laying around my shop as I don't own a 6 inch random orbital sander (mine's a 5 inch model).  Using a digital caliper I measured the thickness of the board and sand-paper together and then added layers of tape at both ends of the board to achieve the same thickness (approximately).  In my case I used three layers of an aluminum metal duct tape.

 On the scarf joint not only did the epoxy need sanding, but the thin edge of the scarf was pushed up or curled up a little in places.  I used the random orbital sander to take down the bulk of the high areas and then used the sanding block to finish.

Using a regular sanding block to level a surface would require periodically stopping to check progress using a straight edge.  This new block checks the flatness during the sanding with not one, but two methods.  First when the block is over a high spot, the ends of the block will see-saw up and down.  The smaller the amount of see-sawing the closer you are to being flat.  Second, when actively sanding you can feel the bite of the sand-paper.  When the high spot is sanded away, the sand-paper no longer bites and the block slides more easily with less friction (being then supported mainly by the tape on the ends).

Now you might say a really long sanding block will also flatten a surface.  But I think that the ends of such a sanding block can't help but sand areas other than the high spots.  This block only has sand-paper in the middle of it to concentrate the sanding on the high spot while 'leveling' to a much longer area.