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How to make a sanding block
Select a wood block.
You can use a small block of off-cut wood retrieved from the scrap pile, provided it's the right shape. A rectangle is the optimal shape because it's easy to hold and covers a good area when sanding.
If needed, cut a piece of ordinary wood into a rectangle of a size suitable to hold in your hand (around 2 " x 4 "). Fold the piece of sandpaper over the sanding block.