Brand new metal will have surface rust if unprotected.
Painters will wipe new metal with Ospho. Then wipe down with an oil like fluid film to prevent new rust. Clean the oil off before painting.
If you can only work a few hours at a time I would work in sections. Do the grind, clean, and ospho treatment in one work session. Then either paint that section (after removing the ospho residue (mineral spirit wipe down) then either (within 30 min!) paint, or coat with fluid film.
Otherwise, like you already mention, your 'fresh, raw' metal will nearly immediately get a new surface coat of rust...
When you do paint, several thin coats will be stronger and more likely to 'seal' the metal than one heavy coat.