I've had the black furniture for a while. It came from my grandparents, who have had it for many many years.
First I emptied it out and cleaned up the slimy rocks (yes, I washed rocks!) Once everything was un-slimy I could put it all back together and run the pump (it had to be cleaned out too - it was FULL of slime and hard water).
After scrubbing off the rust, dirt and cobwebs, I laid down a sheet and sprayed everything with a generous coat of Rust-oleum.
After each piece was dry (about an hour), I was able to put it back on the patio. Even the neighbors were able to notice the huge change in how it looked.
I also went to Home Depot and bought an indoor/outdoor rug to brighten up the patio - I was surprised at the difference it made.
Now on to the bench. Once the rain stops....