Questions for Evaporust Users

I've never used evaporust before and have a couple questions for those who have before I attempt to use it to de-rust the under side of a deck lid (photos below).

1. How long does a part typically need to soak for it to be effective? I realize "it depends," but I just wanted to get a rough estimate for typical surface rust on sheet metal.
2. Does it kill grass??
3. Would sloshing it around an upside-down decklid suffice, or does the panel need to actually be fully submerged in the solution?

My idea was to just lay it upside down, fill it up as much as possible, slosh it around periodically, and hope for the best before rinsing it out.

Here's what I'm working with:



To not answer your question, I will say that your deck lid is in very good condition. I would not subject the rust to evaporust or other liquid rust killers in this situation. I would use por15.