Cast iron does not have a protective coating - so exposure to even moist air can cause rust. To remove rust before painting (View this link to wikiHow)
Cast iron brackets can be painted with any type spray paint that is designed for metal. If you plan on using the piece outside you may want to get a paint that has a rust inhibitor in it.
Your cast iron bracket will rust beautifully by just leaving it outside in the rain for a couple of days. If the rain is not cooperating you can water the pieces a few times a day. This will take a little longer but achieve the same results.
Once your bracket has rusted you need to clean it and brush it off with a scrub brush to remove any loose oxidation.
The last step is to seal the iron. You can do this using any clear polyurethane spray paint. We have achieved good results with Minwax clear satin finish polyurethane spray.
Be creative when finishing your cast iron decor
You might want to try hand painting it.
Or try rusting it and then applying a coat of paint for a distressed finish.