Get the seats out and all the wood pulled up. I found that an enormous pry bar and patience were the best tools.
Mine was pretty crusty in a lot of places, and all the plywood saturated, moldy, and falling apart but no rot or rust holes anywhere. Another year or two it would have become swiss cheese.
They nailed the wood into the steel with spiral shank nails, hundreds of them. Many were rusted so badly they looked like burnt matches. I knocked all those out, too.
Finally, use an air needle scaler to remove the thick rust scale, and see what's left. If it needs patching weld it, don't just use fiberglass body filler. It might be galvanized steel so watch for that.
Once I was done descaling, I went back with a bunch of cup brush and wheel brush grinder wheels, followed by a dressing of fan disc abrasives. Finally, everything was rust converted with like 4 gallons of phosphoric converter. The "kleen strip etch and prep" from home depot is inexpensive and works great. Don't get it on your clothes, skin or eyes and use a resperator.
I mopped that stuff over the course of about 3 days in the summer, and got all the rust I could see.
FINALLY, sweep and clean your surface and seal with paint. If the metal was ever galvanized, do NOT use a oil based paint, use a latex paint. Killz 2 works great.
Oil based alkyd paints react with the zinc to saponify into a complex zinc salt based soap substance. (Alkyds are very basic, just like lye and oil make soaps) at which point adhesion is lost and it becomes a moisture trap.
Latex based paints won't react that way.