So I wanted to do something to seal up my cowl and this is the solution I came up with. I went to Petco and got a plastic small animal medicine feeder syringe. I took rustoleum and thinned it, put it in the syringe, and then started at the middle of the cowl and worked toward the edges. It took some time, but only maybe an hour or two to keep refilling and releasing. At the edges I took spray rustoleum and sprayed the outter edges thru the drainage holes on either side and then sprayed some undercoat on top of that. I'm hoping this will keep my cowl sealed up good and hopefully this'll help some of you guys whose cowl isn't rusted out to keep it that way.

 

Mine only had surface rust also so I used a solder flux brush, they're metal and hollow so you can stick a screwdriver down the center for more reach, they sell them by the bag at HD/Lowe's near the solder.

I used Eastwood liquid rust converter (thicker than the spray) and put painters tape protecting the vent slots every 4"-6" so I wouldn't get conv. on my paint, then dipped and slipped the brush through there and brushed every visible area. You can also bend the brush handle to get up towards the seams. Went through the drain holes also, pretty well covered. I had my heater and drivers floor vent out so I did the hats from the inside also. I may add some POR15 if I have some leftover when I use it again.
Necessity is the mother of invention.
Jon