Cars typically don't rust from the outside in, they rust from the inside out. Unless your car is rusting from unmitigated paint chips, you most likely have a problem that paint is just going to cover up for a short while. Crawl under it, look around any seams and attach points, wheel wells, and make sure there is not rust there. Rust repair is not paint, rust repair is tons of labor hours removing the bad metal and replacing it with good. The paint is just what you see.
If it really is just the hood, try a boneyard and get as close of a color match as you can.