I cleaned most of the cowl on my 79 SW this weekend.
I say "most", because I'm not sure just how much moss, etc. is left in places I can't see. But here's what I did -- I took a long knife, stuck it through the slots on top of the cowl, and cut through the moss and leaf debris -- it was all dry. So I cut it up as small as I could, and then took the vacuum cleaner and reversed it, blowing air out through a crevice tool, and held the crevice tool against the slots on the top of the cowling.
The air from the vac blew out a lot of the debris out through the slots, and all over the hood.... some of it bounced around inside the cowling, being pulverised....
I could see the bottom of the cowl and there's rust there, but it's not bad yet. When I blew as much of the debris out as I could, I sprayed the bottom of the cowl with WD40 to stop whatever rust is there.
The next time I turned on the defroster it showered the inside of the car with leaf debris.
And the time after that, and then again this afternoon.....
It's not a permanent fix, but I feel a lot better about it now.