So here's my question...why keep using wood for playfields? Why not try something else like a hard plastic?
Wood is not very consistent as far as atmospheric conditions. Swells up, warps, splits, it gets those nice dimples in the finish when the ball jumps around, and termites will eat it.
So why not a thick sheet of acrylic plastic. This is where I think Hard Tops has a good idea. I do not work for them, so I am not trying to sell their products, but I did one on my Evel Knievel and I really like it.
But instead of sanding down the old wood playfield, just make a thicker sheet and do a playfield swap like a CPR type playfield. You could still drill out holes for your inserts and the like, and maybe it would last longer and hold up better. No more waiting for layer after layer of clear to dry and all the crap.
I know some will say, "cause that's the way the factory did it" and that's fine, but we also used to use horse and buggies to get around and then we invented automobiles and there was resistance to that as well. But we got past that too.