As said earlier, you only need to 'key' it or scratch it up so the paint can adhere to it. I used some of those foam sanding blocks because they fit in all the grooves of the van floor. Only needs a quick rub over, not back to metal.
Truth is though, I hardly get any water in the back of my van. Only a few drips off the brush head or if I spring a leak which I fix asap.
I found the Protectakote was handy for making the back of the van non-slip though. On wet days, with wet feet, it did get rather slippy in the van.