Once a cast iron planter starts to rust, watering the plants will feed the rust and make it spread. You want to clean up that rust and paint the planter to keep the rust at bay.
Remove rust by scrubbing it with a wire brush. You want to get all of the loose rust off of the piece so you can seal the surface and it won't eat back through.
Once you've removed all of the surface rust, use a rust inhibiting spray paint to refinish the surface of the planter.
Spray evenly working from top to bottom. be careful not pool the paint or you'll end up with drips.
Several thin coats are better than one thick one.
Once all of the paint has dried, try rubbing a coat of paste wax over the surface of the planter to keep excess moisture at bay.
Look at the difference, rust free and protected. Now all you have left to do is fill the urn with some gorgeous plants! I'm Mrs. Fixit and it's just that simple!