How to Make Custom Sanding Blocks

RON HAZELTON:
Now a sanding block is the right tool to use for flattening a board like this.  But what if you want to get that same flattening effect on a contoured surface like this piece of molding? Well that's when you’ve got to think out of the block.

Well the truth is, to create a good contour sanding tool, I will be starting with a block, but not a block of wood.  Instead, a block of rigid foam insulation.  One way to create a custom sanding block is to trace the molding outline or profile onto the end of the foam insulation block. 

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A piece of double stick carpet tape applied to my workbench top is a great way to hold the foam insulation in place while I work on it. 
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I usually extend lines back along the face of the bloc.  This tool called a micro plane is actually a very fine and very sharp rasp.

If it looks like something you've got in your kitchen, that's because the manufacturer uses the same technology to make high quality lemon zesters and cheese graters.  At any rate, kitchen or workshop, it's perfect for shaping foam insulation.

Now to use my contour block, I'm just going to wrap some sandpaper around it.  Leave it a bit loose here on the bottom so that it can conform to the profile.  Set it on the molding and begin to sand. 
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Now if you don't happen to have a set of micro rasps or you just want to try a different way of shaping that rigid foam insulation, then consider this - it's something my neighbor Rick showed me. 
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Take a scrap piece of molding identical to the one you want to sand and a piece of sandpaper. Apply spray adhesive to the back of the sandpaper and to the molding. Let it get tacky, then stick the sandpaper on the molding, grit side up. 
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Use a putty knife to press the sandpaper into the grooves and crevices. 
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Then take a foam block, set it squarely on top of the molding and begin moving it back and forth.  The molding shape will transfer to the foam.  Now I have an exact duplicate of the molding profile.  Once again, I could just loosely hold the sandpaper on here or I could do this. A little more spray adhesive applied to the contoured foam block and the back of the sandpaper will allow me to attach the two securely and eliminate any chance of slipping.

Finally I trim off the excess paper and I'm ready to sand. 
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Now I ask you, what could be easier than this?  With a piece of rigid foam insulation, some sandpaper and spray adhesive, I can make a custom sanding block that will fit just about any piece of molding.