Wire brush the loose rust, clean with IPA, then paint 2 coats of POR-15 High Temp Paint with a foam brush:
https://www.por15.com/HIGH-TEMP_p_104.html
I've done numerous cars from daily drivers to garage queens. This is a daily driven GTO that I did over 5 years ago. It's owned by my father, sits outside 24/7, and still looks perfect to this day: