We have an old Steinway B soundboard pattern made out of masonite. It will be trimmed and modified to fit the new Steinway B rebuild. Woo, the old pattern almost fit in the "new" B. Mark where to trim, sand it down with a belt sander. Ware a mask for that because of the very fine dust out of masonite and old epoxy.
Once the pattern fits in above the taped ribs, slightly smaller on size, smear a thick layer of wax on the tape and onto the side of outer rim. Fitting with ribs in makes sure that the pattern is exactly the size of soundboard that we'll trim later.
Mix up epoxy resin, hardener, and filler to the consistency of mayonnaise.
With the pattern pressed down under go bars, apply epoxy into gaps around the pattern. A syringe with a plastic tip works well here. The wax on tape and outer rim prevents epoxy from sticking onto rim and ribs.
The next morning, epoxy is dried. Take the pattern out of piano, sand around till flat. clamp pattern onto soundboard, make the first cut with a jigsaw, 1/8" away from the pattern. This step reduces workload on the router and operator. Set router bit bearing height to ride along edge of masonite, trim soundboard flush.
This method makes a well fitted soundboard. Worth the extra steps. The next will be mark and glue ribs to soundboard. Now everything will rest in kiln to keep the moisture content down at about 5% before the big gluing mess.