I'd have to guess that the Sunderlands were designed from the ground up to be more tolerant of salt. Things like avoiding contact between dissimilar metals where possible, using more corrosion-resistant alloys, avoiding closed areas of the structure that can trap salt, etc. None of that could completely prevent corrosion, of course. But if it came down to something sprayed on, they could have just loaded the salt in a C-54 and sprayed it afterward (and/or before).