I would be extremely wary of any planking, which I seem to have seen pretty evident in a few places. Just painting over the existing ink without sealing it is just asking for trouble; although if you are keeping the machine in a temperature AND humidity controlled environment, it will last longer than without it, but the playfield will continue to wear, regardless.
From what I've seen though, you've done a bang-up good job of restoring it to the level you currently have it! 
My Spirit playfield has been disassembled since 2011, when I sent my main playfield out to a popular restorer (so-called) who completely ruined it. Only the clearcoat on it is decent. The repairs and touchups plain suck. They even filled in (with wood filler) a post hole that is SUPPOSED to be on the playfield.
Now, I'm waiting for Vid, or HEP, or maybe even Kruzman to repair the "work" that this other so-called restorer attempted, but failed miserably at.
Getting one of the pros to fix it has been a damn rollercoaster ride so far, also.
I hope I don't end up parting the damn thing out.
