Paint on wrought iron tends to chip. With black over white, the defects will stand out visually. You can prevent this from happening if you remove the wrought iron railing, and get it sand-blasted to remove all the white paint. The black finish should be applied professionally, using the toughest coating available. This would be done in a paint booth, at a shop that does commercial painting. They would probably use a two part urethane that is resistant to chipping.

I have lots of wrought iron, and this is what I found to be the most effective. I use wire wheel attachments on my cordless drill to take off any rust, obvious chipping and other rough patches, sand lightly with a sandpaper foam block (it forms better to the different kinds of surfaces) to rough up the painted surface, dust and clean well, then paint with a good primer and three top coats.

I use Rustoleum in the spray cans, but I my iron is mostly outside. The spray cans are more expensive and messy, but I cannot duplicate the smooth surface no matter what kind of brush I use.

I have also used spray paint indoors, but only after constructing an elaborate "tent" from boxes and plastic bags, and opening all the windows and covering everything within reach. Using spray paint this way would be the most timeconsuming part of the process, but worth a test run on a small section and comparing it with a handpainted section.

If you decide to handpaint it, foam brushes work best. I use the new kind that are pads with a silky "fur" on the top...they make small ones meant for window sashes, but it can be time consuming and you can go through a lot of pads (they're cheap though).

And if the new paint chips or rusts, it's a simple matter to touch up with a little sandpaper and paint.

Hope this helps.

PS: Also use plenty of protection -- My house was built in the 60s, and I bought it from the guy who built it, who left me every paint can he ever used on/in the house, so I have no lead paint.