It's an etchant. You don't leave the railings in their for hours, a minute or two is usually enough, just so that the finish of the parts looks totally dull and matt, then rinse of in fresh water, degrease and paint. The softer the metal, the less time it needs, for instance white metal fittings only need a few seconds in the acid to turn them from silver to a very dark grey, almost black.
The etchant will put millions of microscopic craters in the metal giving the primer something to cling to. You won't see this in the painted finish. Try it out on some scrap material first, if you're nervous, but you needn't worry, you won't dissolve your parts unless you leave them in overnight or something silly. You can keep the etchant in a jar and reuse it over and over.