The first piece of armor that I made, was the bracers (which are armor for the forearms).
The idea was that I would make a full size clay model, create a rigid mold, then cast the wearable pieces in urethane casting plastic.
The general "clay model -> mold -> casting" idea was sound, but the mold material choice wasn't.
I'm using mostly products from Smooth-On. The first mold, of the outer bracer, was made using Shell Shock® Brushable Liquid Plastic as the impression layer, with a support backing of Plasti-Paste™ II Trowelable Plastic. The reason for this was that before I knew better, I got Roma-Plastilina sulfur modeling clay, and I was worried that a silicone mold rubber wouldn't cure against the sulfur.
What ended up happening is that, despite my using both a paste-wax and spray release agent, the mold stuck to the first casting. What a pain! It took me several hours to crack and chip away the mold from the cast. Luckily the casting, made from Smooth-Cast® 326 ColorMatch® urethane casting resin with URE-FIL® 3 ceramic filler, is TOUGH! It was able to withstand a suprising amount of abuse, like chipping with a flat bladed screwdriver and small hammer, while I got the old mold off.
So when it came to make the inner bracers, and to make a new mold for the outer bracers, I switched to using brushable Mold Max® STROKE silicone rubber for the impression layer, instead. This stuff is great. Because it's self-thickening, you can very easily form a suitable thickness mold around the original, without wasting tons of rubber (and thus money) if you tried to pour a boxed-mold around these 3D objects. I've had no trouble with the silicone not curing, even directly against the unsealed Roma-Plastilina brand sulfur-based modeling clay. And best of all, the silicone doesn't stick to anything, and needs no release agents.