I make a special effort to keep the cavities protected using fisholene , there are several other waxy, penetrating, preservatives that can be sprayed in the cavities and allowed to soak into the seams. paint won't do it. I have been fortunate that reapplication each 5yrs seems to be enough here. Also regular hosing under the fenders and a good coat of underseal (also checked and reapplied as necessary) seems to work ok.
Bill makes a good point about accumulated debris in the cavities. I have made a point of enlarging drain holes and flushing the cavities, blowing dry with the vacuum cleaner, then the fisholene etc.
It all takes a while
but Ive used my truck daily for the last 15yrs and all's well.
ps. I did the same on a '56 Belair that I owned for 10yrs, it also survived well enough that a subsequent owner exported it to Australia (it was in such good condition).
