Plus40,
I think you may be going to extremes here. If you take a block of wood or other hard object to back sandpaper in order to TRUE a ball, you may not be able to do that to a ball more than once. Additionally, it makes the sandaper contact the ball at one very tiny point, which is more likely to create flat spots or areas; besides, you will be wearing away far too much material from the ball.
With a spinner, most drillers, before the advent of machines like the Haus machine used their hands, quite a "spongy material", did not do a bad job of re-surfacing balls. Besides with all the flaring on balls these days, most balls do not get very compressed on their track, like the old plastic and urethane days, where the track never moved. I think that only plastic spare balls' tracks get compressed and out of round these days.
I have to believe that a spongy material like a damp sponge behind both sandpaper and nylon pads is a good thing, not a negative.
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"We get old too fast, and too late, smart."