On my '66 it looks like the design flaw is the "pocket" where the back of the fender meets the mounting bracket. Wet dirt & mud is kicked up by the tires and flung there, where it sits.
I live in the desert, so I'm not concerned about fixing this flaw when I replace my rusty fenders. But for those in damp climates, it looks like the solution is to weld an inner wheel-well in that spot so all debris is kept out.
Hope that helps.
---Scoob, Tucson
