I used that stuff on the floorpan in my dodge, it will only work if you get it really clean. It sprays on clear, and dries black so you will know if it worked or not. Problem is you have to get it so clean, you may as well just sand the rust off to begin with. I won't be wasting my money on that stuff anymore. I did my whole floor, and only had a few pebble sized spots of black appear, otherwise it didn't change anything. Skip the cheap fix and do it right the first time like the others recommended.
1967 Mustang Coupe 351W/4spd
1993 F-250HD SC 4X4 7.3TD/5spd
2012 Jeep Wrangler Sahara 3.6/5spd-auto
1979 Mercury Capri 302/5spd-Sold(found and being restored), Ran 12.47 @ 110mph