It's gonna get scraped up and dirty anyway. I'd do like the others that say just sand or wire wheel the loose stuff off, and hit it with the right color of rustoleum or some other brand of rust paint.
If you really want to get the whole thing clean one time, take it apart and power wash it. Take some 2X12's and build a box big enough to hold the parts. Line it with heavy plastic. Put the parts in setting on some wood blocks to keep them off the bottom.
Fill it with water and add some washing SODA (not powder). About 2-3 pounds should do it.
Attach a wire to each of the parts and connect that to the negative lead on a heavy duty battery charger or DC arc welder.
Put a piece of angle iron in each corner of the pond and connect those to the positive lead of the charger/welder.
Turn it on and wait about 5-10 minutes. You'll see bubbles start to form on the parts. The rust will leave the parts and attach to the sacrificial anodes (the angle irons). Let it go overnight and check it the next day.
Turn off the power and unplug it! Pull a part. You should see a nice, black part. Dry it off, clean it up lightly with a wheel, and paint it.
Do this outside, by the way. The little bubbles are hydrogen gas and it can explode!
Have fun! :thumbsup: