I had the problem of water leaking into my grandfather's wagon, and found it to be a huge nest (chipmunk I think) in the cowl. The car had been sitting outside and started to have a small leak into the car. Considering it probably was blocked for years and how much gunk was in there I can't believe more water wasn't pouring in. At any rate, the idea of using rustoleum is what I did as well. I did it a little different but took an animal medicine dropper and sucked up rustoleum and then sprayed it down through the cowl to cover the area. Took a little time, but it worked well. I should mention that I did the rustoleum in a '66 Mustang I restored that did not have a hole in the cowl.