The thought just crossed my mind whilst thinking of a Godly Pledge, I
cannot remember if the requester of cleaning techniques mentioned the
soundboard finish on the instrument in question!
If the soundboard were left in egg white size I would have thought any
form of moisture would not be a good thing. Indeed that would surely
be the case with a soundboard sealed with glue size as used by Owen
and many others. Should the soundboard have been sealed with some
form of shellac, then a slightly damp cloth would not harm providing
the strings are thoroughly dried after the cleaning. I remember back
in my John Broadwood days we used to revitalise a soundboard before
re-stringing with a methylated spirit rubber. All soundboards in
those days were varnished with a shellac varnish know to the UK trade
as White Hard Varnish.
The vacuum cleaner and a brush are usually all that is needed I think.