I have done a number of wood-to-iron stair baluster conversions. Some were where the old turned wood balusters are upgraded to wrought iron and the existing railings were used. This process is nice and easy when the spacing and hole locations remain the same. Others were where 2 x 2 outdoor deck type balusters were removed and new railings were installed; posts and rails needed to be installed and new mounting holes drilled.
In most cases, wooden posts are added to the mix to provide some design continuity with the wood hand rails. I mount these wooden posts with a special mounting kit that uses “L” shaped metal brackets that connect the post to the floor with some heavy screws, a special wood trim kit covers the metal brackets. In most of these cases the posts and handrails are installed sequentially.
Once the handrails are in place, the wrought iron elements are installed. Here the spacing is determined and holes drilled into the bottom of the handrails and either the flooring itself or a newly installed lower rail. Some railing manufacturers recommend the use of epoxy to firmly set the iron, but I have found that adhesive caulk works just as well. It takes a bit longer for it to fully cure or set, but is much easier to work with and clean up. The holes in which the iron is mounted (top and bottom) are covered by additional trim pieces called “shoes”. These shoes come in standard flat profiles for horizontal runs and also angled versions for use on stairs. The shoes have a recessed set screw that keeps them in place.
Some folks may be intimidated working with metal instead of wood, but a thin bladed metal “cut-off” wheel mounted in a chop saw or hand held grinder can make quick work of cutting this material to length. That, and a simple spacing layout, can transform a railing or staircase in no time.