Here you go, easy to understand,courtesy of Mr Boeing
http://www.boeing.com/commercial/aer...7/corrosn.html
and a picture too
http://www.boeing.com/commercial/aer...osn_fig05.html
Corrosion is difficult to treat, the best method being complete replacement of affected parts; any metal
repairs, (cutting out bad and joining in good) will have to be done iaw manufacturer or type cert holders recommendation as listed in the Structural Repair Manual or by a one-off request.
The pre-buy surveyor is probably giving you the best advice, shop elsewhere unless you want to end up paying more than originally budgeted.
ttfn