Sanding Shims
My father and I didn’t start out using sanding shims when we created large intarsia projects for commercial installations. We would sand one piece, put it back in place and mark where it joined the next part. Then try to sand the next part down to the line as close as possible. It was years later we came up with the idea of sanding groups of pieces together as one unit.
That's me under the hat, mask, and eye protection. We didn't have a dust collector at that time.
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The concept “sanding shim” is a term I use to describe a method for sanding many pieces of wood together. I cut a piece of flat plywood or masonite roughly the shape of the parts that I want to sand together. I use Light Traffic Double-Sided Carpet tape to hold the parts to the masonite (or plywood). This is temporary, after the parts are sanded they are removed from the “sanding shim”. The raising shims are a permanent part of the project.
When I shape the pieces my goal is to make it look as real as possible. For example if I were sanding a horse with a white blaze, a color change on the coat does not mean the contour changes. After the parts are sanded they are removed from the sanding shim and I hand sand a very slight bevel around the edges of the parts.
Sanding shims comes in handy when you need to blend raised areas. Using the raccoon as an example, it has two raising shims under the nose area to make it more dimensional. The raised nose needs to blend in with the forehead, so it doesn't look like it has a stair-step shaped nose. I use double-sided light duty carpet tape to hold the parts onto the sanding shim, making sure the raising shims are taped in place before sanding.
I use an inexpensive “light duty” carpet tape. You will need at least two pieces of tape to hold each part down. It is best to put the tape on the back of the parts, it is easier to see where to put the tape. Otherwise you practically have to cover the entire surface of the sanding shim with tape to ensure each part has tape. I really do not like to use that much tape, it makes it much harder to take the parts off the shim without breaking anything. 
If there are more than one raising shim stacked up it is going to be easier to put the tape on each raising shim and the sanding shim. Then you have to carefully assemble the parts making sure the raising shims don’t interfere with the placement of the parts. The trick to this is not pressing anything down firmly until everything is in place.
If you have trouble keeping the parts on your sanding shim there may be several reasons.
One problem is the sanding shim is not flat or the parts were dusty and the tape did not make a good bond, or there isn’t enough tape to hold the part down, and last but not least perhaps you are pressing too hard against the sander.
An example of parts taped on the "sanding shim" starting to "rough in" the project. Taping all the parts to the sanding shim not only saves time but it will make the project look more realistic. 
The giraffe is a perfect example.
As mentioned above this technique makes it easier to blend raised areas. Another benefit is being able to see the entire face as you sand the surface. After the parts are roughed in they are taken off the sanding shim.
We see many intarsia projects that have each part heavily rounded. It gives the project a quilted feel and accentuates each part that makes up the picture. There is nothing wrong with this, it is more of a personal sanding choice that each person must decide how they want their projects to look.
As mentioned above I have a realistic goal in mind. I think if each piece is heavily rounded the first thing you notice is all the different pieces of wood.
If they are sanded realistically you see the completed piece and then you realize it is made up of many pieces of wood.
Another thought to keep in mind is balance. It can look redundant to have every part heavily rounded. To balance the project I like to have some mellow contours to help accentuate the more rounded parts.
On the Big Foot Clown, the fingers really stand out next to the flat soles of the shoes. The hair is more flat to help make the face look more dimensional. The face has a mellow contour to help make the nose stand out more. The same with the hat, it has a mellow contour, which makes the ball on top look much rounder than it is.
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Judy Roberts
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