Noticed you hadn't really gotten an answer about using the Ospho.
I did my floor last week and put the first coat of Ospho on my entry steps this morning.
It's the same consistency of water. I originally put some in a small plastic paint cup and brushed it on with a regular 3" brush. Easy.
For the second coat on the floor (there were some areas I wasn't sure got converted) as well as the entry steps I just poured a bit out (making sure you don't splash yourself or anything else "fragile") on the rusty areas and spread it around with the paint brush. Leave overnight, brush off any loose crumbs the next day and paint.
The stuff stinks something awful once it starts doing its magic so I suggest a respirator or at least an open wide bus with ventilation. Rubber gloves are highly recommended too.