So here's the problem. This mandolin's action measures 1/32" too low at the 12th fret. It has a nonadjustable bridge:
The top of the mandolin has a slight arch and the bridge fits that arch perfectly.
My job is to raise the bridge 1/16" so that the action will be 1/32" higher at the 12th fret. Generally, I like to glue a shim to the bottom of the bridge to raise it, rather than having to make a new taller bridge.
First, I'll protect the top with a wide piece of low tack sign makers transfer tape:
With the tape in place I can fit the bridge without injuring the top of the instrument.
Notice that I'm leaving the strings on, but I've loosened them a lot so I can easily lift them up to remove and replace the bridge.
I'll spend a couple of minutes to make a 1/16" thick shim at least 1/2" longer and about twice as wide as the base of the bridge. Naturally, I'll use material similar to that of the bridge (ebony for this one.) I use my 1"x42" belt sander to make the shim uniformly 1/16" thick.
Next, I spray a nice wet coat of cyanoacrylate catalyst (accelerator) on my shim piece before placing it on the mandolin top where the bridge sits:
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