Re: 1966 mustang coupe !!BODY!!
should be repop cowl available as they were common to rot. on bare metal I would STRONGLY recommend putting metalprep on immediately- sandblasting will leave a surface that rusts overnight, and sand will get into hidden nooks and crannys that wont dry properly...metalprep is weak phosphoric acid that will attack any rust...converts iron oxide into stable iron phosphate coating that wont rust...this will prevent paint blistering from underneath...in humid areas, you cannot paint fast enough after sanding/blasting to prevent microscopic rust from starting first. phosphoric acid etches the surface/stops oxidation in its tracks. use lots of water to rinse. then use a lot more. then even more. once youre sure all acid has been washed away, be sure to rinse again :)
guy at work showed me this stuff long ago...took a piece of new sheetmetal, scotchbrited it, wiped metalprep on one half, let it sit a few minutes, thoroughly rinsed it off and sat it outside wet. came in next day, and untreated side was orange. other was still shiny. very cool stuff.
Confidence: that feeling you have just before you realize the mess you've gotten yourself into...