"blotchy" soundboard

[pianolover 88]

I'm restringing my customer's 1926 Wurly Baby grand. After removing all the 
old wire and pins, I began the lengthy cleaning/detailing process. This had 
to be just about the dirtiest, dustiest piano I've ever encounterd...and 
i've seen some pretty bad ones. Anyway, After removing and vacuuming the 
loose debris from the soundboard, i started cleaning the decades of dirt and 
grime away, and started to notice that certain areas were getting "brighter" 
and lighter than others, although I was using uniform pressure on the board. 
I used (as I always have with excellent results) Howard's restore-a-finish, 
neutral color, with 0000 steel wool. So now I'm left with a very Blotchy, 
light/dark soundbaord. I'm not sure if the remaining dark areas are just 70+ 
years of dirt and grime, or whether it's supposed be dark, and that the 
lighter areas are the result of too much rubbing...I'm pretty sure  it's the 
former.

I need some recommendations as to what would be the most effective cleaner 
that would most easily cut through the layers of gunk to expose the original 
"honey" color?  I was thinking about Murphy's Oil soap, but I'm guessing 
that won't be strong enough for this job. what about some kind of solvent 
that will quickly break up the grime, but not harm the wood, or varnish, 
what's left of it anyway.
Also, Any tips for cleaning between hitch pins? I'm afraid that a wire brush 
will scratch the plate; and a paint brush won't remove the almost black 70 
year build up; solvent there too? Thanks list!!!
Terry Peterson