Not replacing roof or cutting out rust. What type of sandpaper to clean up drip rails enough to apply flex seal clear only around drip rails? Short on $$, need to stop leaks before rain.
Start with heavy sandpaper, like 60 grit. Fold it in half, but try to keep the fold thick. Maybe wrap it in a pencil. Then slide that fold in the drip rail (without the pencil), and clean as much rust as you can.
You can repeat that process with 120 or 180, and if you want a prettier end-result.
Then I would get some "End Rust", or similar product that chemically attacks the remaining rust. Those products neutralize the remaining rust, preventing it from spreading, and leave a light protecting coat that you can paint over. One type comes in a "rattle-can", and works very well. The rust turns from red to black after the chemicals attack it.
If you have holes that leak, fill them with expanding foam (Touch-N-Foam, or similar). Shave back the foam that expands out too far, seal it off with bondo, and prime it so it's somewhat weatherproof.