Can you remove the steps? If so, there is a product I have used “Evaporust”. It has worked well for me. I place the object in a bucket full of Evaporust, and let it sit over night, wipe it down. Heavier the rust the longer it needs to soak. I have had to have an object soak for 24 hours.
I have seen people place an item in a parts washer, with good luck. I have also seen people take a kiddy pool and make a plastic tent over it, using a pump connected to a sprinkler to keep the metal wet.
I bet if you wetted down some rags and wrapped the steps and cover it in a plastic wrap (to keep it from evaporating) it would also work.