Remove the old spindles by cutting the wood or iron in half with a jigsaw. Fit your jigsaw with a metal cutting tool if your balusters are made of metal. Pull on each half of the baluster to remove it from the railing.
Reach into the old holes left by the spindles with pliers and remove any loose pieces of wood, iron or adhesive. Clean out all of the debris from the holes.
Measure the hole with a tape measure. Iron balusters 1/2 inch thick will need a 3/4-inch-wide hole. Drill the holes until the bottom hole is 3/4 inch deep and the top hole is 1 1/2 inches deep. Use a hole saw attached to your power drill to make these holes.
Measure the distance from the top of the stair rail to the bottom, including the depth of each hole in the railing. Trim the wrought iron baluster from the bottom to match this measurement. Use a circular saw and a metal cutting blade.
Combine the two-part epoxy according to the package instructions. Insert the epoxy in an epoxy gun. Squeeze the trigger of the gun to fill a hole in the bottom rail.
Put spindle shoes on the rail so they will sit properly on the baluster once installed. Spindle shoes are caps that rest on the top and bottom of the baluster to cover glue or gaps where the baluster meets the railing.
Slide the top end of the spindle into the hole in the top rail. Slide the other end of the spindle into the hole in the bottom rail.
Secure the spindle shoes to the bottom of the spindle with the included screws. Let the epoxy set.
Squeeze epoxy into the gap between the top of the spindle and the stair rail. Allow the epoxy to cure. Slide the spindle shoe into place and secure it with the included hardware.