If it doesn’t need to look good, I have a solution that works for me. At Walmart in the automotive section, they have a product called “Rust Converter.” Basically, it converts rust into a primer coat. Scrape off as much rust as you can with your fingernail, lightly dab a little rust converter, and let it dry before you add the touch-up paint. You might consider a second coat of the rust converter before you paint. When you add the touch-up paint, carefully dab the brush on the spot, being careful not to use too much, and don’t expect it to look decent on the first coat. Let each coat dry for 30 minutes (or longer, depending on the directions on the paint container) before you apply the next coat. It won’t be perfect, but the metal will be protected, and it won’t stand out much if you do it right.