How many people have actually sanded the converted rust spots that now appear black? Can't speak for Ospho, never used it, but other rust converters I've tried, whether it be other naval jelly or even zinc phosphate turn what rust spots that aren't dissolved black. If you go sand those black spots after they've been 'converted,' you hit more orange rust underneath. That just doesn't sit right with me so I mechanically remove all the rust first if it's just surface rust. Then I'll put epoxy over it.
Also, I'm just a hobbyist, not a pro.