I used a marine grade resin originally and it SUCKED(read it as EXPENSIVE stuff that takes days to cure)but the last set I did I used Crystal Clear® 220 by smooth-on not as expensive and seems to work as wellor better by having a lot shorter curing time and I think 8900 tensile strength. it also has UV inhibitors in them . I cast some for my friend in his 65 plymouth B body and it's been going for 4 years now and still looking good. since I have the molds for the lenses only for the 66 dart (going to refine the shape a little bit better and re-mold the lenses and bezels together in red) since it'll cost me darn near a grand to re-chrome the bezels I thought I'd take my time and cast both the bezel and lense together for a solid red unit that I can just bolt on to the housing. I still have to join the 2 pieces per lense and cast them, then fill and sand any pits, paint it until it's mirror smooth and THEN cast the complete unit. My cores/original units have bombshell type pitting/craters in them so it'll take some time.
heck you'd think as cheap as chrysler was that the left inner and Right outer would be the same and the oposite way too, but NOOOOOOO they had to put a darn crease in the trunk and make it hard. I have to make 4 seperate molds because they are all different, not much but enough that you can't cheat by making one side and copying it to the other. same way for the led board 4 different ones there too.
since I have a lot more time than money right now I just putz around and "git r done" slowly