Home depot has lots of stuff. Get the "kleen strip prep and etch" its in a gallon jug and is green. After you have mechanically removed thick rust, wire wheeled, flap disc grind, hammer whatevered the rest, there will still be rust left over. That rust you can then chemically convert. I use a very stiff bristled plastic brush and a bucket with rubber boots and a respirator + tyvek painting suit.
For the 40' bus I applied approximately 3 gallons over the floor and about 1.5 gallons of the stuff in the basement storage over a few days.
I bought an enormous bag of baking soda from cash and carry, which was cheaper than the soda blasting media I usually use.
After the acid has had time to work (8 hours with some heat) I do an initial rinse with hot water and baking soda in a mop bucket mix ratio about 20:1. I'll alternate between that and regular hose water until I don't get any more fizz then just a few more rinses with hose water.
I immediately follow-up with an electric leaf blower to push the rest of the water out and then heater to ensure its dry.
After that process a few times its rust free and ready for a water based primer since the floor was previously galvanized.
Originally Posted by lizzabus
I understood from some other posts here that Ospho was good to use on the rust after you knocked the loose stuff off.
I just ordered some from Ace in Silver Lake.