minivans have this problem as well. [at least the 3rd gen, 99-2000, and the 4th gen, 2001-2007]
my opinion on this is that there are not enough drain holes in the front underside of the hood, allowing water to remain in place on the uncoated surfaces.
the way the hood is designed, driving in the rain allows water mist to enter the large holes in the forward under-structure, then eventually making it's way to the area where the upper and lower hood structures are crimped together. rust eventually forms, then paint blisters appear.
as i live in the rust belt where a salt brine is used for a winter de-icer, i have drilled extra drain holes in the forward area mentioned, then have my buddy spray his "secret sauce" oil liberally into the large holes of the under side. this stuff makes it's way into this uncoated area. it sure is messy for a while, but after almost 10 years owning this 1999 minivan, i have zero rust issues ! [knocking on my head for good luck ! ]