The easiest and most effective way to remove paint on wrought iron is to sandblast it off. This is not, obviously, a do-it-yourself job. However, you can take the chairs to an establishment that can do the sandblasting. If you want to do this yourself, you have two options. The first is to remove the paint with grinders and steel brushes. On a flat surface this is a good method, but with patio chairs and their nooks and crannies, it will be difficult to remove the paint. An easier, and more effective method, is to apply a paint-stripper gel that removes paint faster. You can use a liquid paint stripper, and the advantage of using it is that you can recycle it. This will eliminate the need to buy a huge supply. If you use a large bucket over which the stripper is applied, it will collect in the bottom of the bucket. Once it has settled, the paint that has been successfully removed will float to the top. You can skim it off, and underneath is the stripper free of any paint, which can be used again. As far as painting afterward, there are many good paints that you can use, but make sure that you first apply a rustproof primer.