Re: how to mold for a resin casting....
First, a couple of questions....how big are the parts ? ( perhaps you can post photos of the item in question?) Next, if it is railing, would you be able to perhaps scratchbuild the item in brass ? I'm not sure that railing or any exposed delicate part is best suited for resin....
That being said, there are a few ways to go about reproducing a part in resin. For something small, it is quite possible to use an 'off the shelf' tube of 1 part RTV silicone, such as silicone caulking to create a very simple 1 part molld - if the part is small and not overly 3 dimensional. It can escalate quickly from here though !
If the part is larger or more complex, you may need a proper mold making silicone, such as Silicones Inc GI-650. A two-part silicone, and while possible to make decent molds without vacuum degassing, it is always best to degas the silicone after mixing, before pouring. I did see in the articles section here, a fairly simple tutorial about mold making.... At any rate, getting into something more detailed about setting up quality molds and casting processes is something beyond a simple reply here, and if you should desire a detailed overview of this, feel free to contact me via message. Some years ago, I had a shop and was producing kits and parts for a number of smaller companies. In addition to resin casting, I also did pewter production castings..... I hope to one day return to that at least on a part time basis. I might even consider a mold making service that would provide the end user with a quality working mold they can then use to make their own castings from.....
I do think that a number of folks might benefit from a more detailed and comprehensive article about the process and at some point in the future I will write something up and submit it for review. Unfortunately all of my equipment and such is put away and will likely not be useable for many months yet, as I am preparing to move.
-Joe