Fresh Air Vent Rust

I would use brush on undercoat (top and bottom) until all the holes are filled. Then I would cut a piece of metal to cover the hole and apply one good heavy coat of undercoat (on top) to glue the metal plate over the hole. Once it cures undercoat the whole thing one more time. At first you can use duct tape to cover the holes from one side so that the undercoat doesn't drip through to get them to seal up. My 2 cents!!!