Back in the days when I was working a lot of gigs I used to put my collection of harps in the soapy bath with me, give them a good workout, rinse under the hot tap and then tap dry on a towel. With that treatment those harps lasted for years - they admittedly were Marine Bands, which at that time were the best stayers. With one exception I never had any problems with the wood swelling (I suspect not a problem with today's harps.

Regards
John