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Select the existing baluster that is in good shape and can be used as a model. Begin by restoring one side of the baluster to re-create the missing details. Use polyester filler to reshape the surfaces. Use modeling clay to finesse the details (Image 1).
Once the column surface is in good shape, one of its sides will be selected and used, along with wooden panels (or "fences") to create the template for a single side (i.e., one quarter) of one column. Four of those will be used to create a final model of one complete column. The final model will be used to create a mold that will then be used to cast the actual concrete columns.
Apply shellac to one baluster side (Image 2).
Install the fences at 45-degree angles to create a mold of that side. Seal the fence edges and baluster with modeling clay.
Apply shellac to the fences (Image 3) and clay and let dry.
Apply molding plaster in a thin layer and let dry. Apply six to eight successive layers of molding plaster and fiberglass.
Glue support boards into the mold with the wet plaster and allow the mold to harden. Remove the mold from the baluster and fences. This first mold is called a "plug mold" (Image 4).
Apply fences to the plug mold to make a container for wet molding plaster.
Pour wet molding plaster into the mold (Image 1) and allow it to harden. Remove the wooden fences (Image 2) to reveal the plaster casting.
This first casting is one side of our four-sided plaster baluster (Image 3). Repeat the wet-plaster pour three more times to create the other three sides of the plaster baluster.
Glue all four plaster sides together to create a complete baluster model (Image 4).
Finesse the corners with wet molding plaster and finesse any other details on this model.
Using the restored baluster model, repeat the first fencing-up process and create four new plug molds. (It's possible to make two opposing sides at the same time.)
Check the new plug molds. Finesse as needed and make sure the corners fit perfectly.
Coat the new plug molds with shellac and let dry
Wax down the sides of this new mold and assemble all four sides to create the final baluster-mold.
Note: Once you have the final baluster mold, you may discard the final baluster model since the model's only purpose was to create a clean and perfect mold.
Now that the process of creating a mold is finally complete, the process of actually pouring the concrete to create the replacement balusters can begin. Compared to the multistep process of creating a model and mold, this part of the project is straightforward. Of course the time it takes will depend on how many pieces you have to cast. In the case of our project house, casting the first baluster was just the beginning of a long, arduous job.
To create the first baluster, wax down the inner surface of the mold so that the concrete won't stick to the mold. Close off the bottom of the mold with a waxed plywood board.
Pour a medium-wet mix of concrete into the mold. Allow the concrete to harden.
Disassemble the four sides of the mold (Image 1) and reveal the first concrete baluster (Image 2).
Let the concrete cure overnight. Finesse the corners and gaps with a similar mix of wet concrete.
To make multiple balusters, simply repeat the process as often as necessary: clean the mold, re-wax it and pour in fresh concrete. For the project house, this process was repeated 99 more times to replace all 100 original balusters.