What is the approximate quantity of rust converter needed to apply on an area of about 11,840 sq.ft, and how much will it cost?

At 1.0mil dry thickness, you can cover about 1,000 sq feet. Any channels or brakes in the metal tend to collect additional material and affect the coverage. If your surface is perfectly flat and you are using a sprayer, we would recommend approximately 12 gallons plus a 10-15% overage, to cover any product lost in transfer, drift, or pooling.