Remove as much remaining soundboard material as possible with chisels.
For pianos not being refinished, use rags soaked in hot water to loosen the old glue, then remove the rest of soundboard residue carefully. Three hours on this piano.
For pianos going to be refinished, use steam on wet rags to loosen the old glue, then remove the rest of soundboard residue carefully. The steam will damage the old finish. But it's going to be stripped anyways. One hour on this piano.