Those nooks are crannys kill your tin!
The best thing to do I suppose is seal it up good with oil, tar, whatever works. I've never done that but maybe some day before I die I'll get the hint and actually do what I know I should???
In the meantime I do know that some nooks and crannys can be blasted out with a pressure washer but the key is to know where they are and figure out access to that spot. The example I have in mind is on pickup trucks- ALL of them in this rust-belt region rot out above the rear wheelwells, because dirt/gravel and whatnot all pile up on top of the inner fender and nobody thinks to blast that out of there. I've been hosing that spot out since my truck was new in '94 and that is one place on my truck that isn't rotted out- but other trucks the same vintage in town are. Brand doesn't matter either in this instance. My truck is suffering rotted rockers under the extra cab and I found out that it occurred from the INSIDE out due to chunks of sound deadening foam stuffed in there that hold condensation.
People would probably wonder what I'm up to were they to watch me wash a vehicle. Most spend all their effort on the top side making it look good while I concentrate on the wheelwells and underside bits.