varying the ratio on JB Weld or other epoxy

I want something like epoxy but that will not dry hard and brittle. Tried the standard 50/50 mix with JB Weld on a piece of cardboard and it cracks when bent, but I understand that this is way it was designed.
On another test piece I put 75% hardner and 25% "steel", then next to (not touching it) did a spot of 75% steel and 25% hardner. 17 hours later neither one seems cured.
The 75% steel spot did the best, it is tack free and flexible, feels sort of rubbery. This is almost exactly what I need if it stays like this over the long term.
The 75% hardener spot feels tacky and has not "skinned" yet.
I know that it's a chemical vulcanization so I expect the cure time on both to be longer than a 50/50 mix. Am I wrong for expecting it to cure at all?
I'm generally curious about the possibility of varying the ratio on 2-part epoxy's, but also am specifically looking for something that dries softer/ more flexible than jb weld but harder than latex or silicone caulk. Doesn't have to be extremely durable, just maintain cohesion and at least moderate adhesion. Also would be nice to have the following:
- post cure service temp of -40c to 110c - curing process should not corrode soft metals like copper or solder (like an acetate or ammonia curing silcone RTV) - not trap small particles after curing. Example - sprinkle some graphite dust on white cured caulk and it will get stuck in the silicone - not "set up" quickly. I need about 30 min. to smear it around.
Purpose is sort of a special electronics project. It will cover and protect some silver based conductive paint on a circuit board. I know it helps to know the application but that's about the best I can explain it. Appreciate any help.
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Devcon makes a product line called Flexane that cures into rubber. They have various durometer ratings, and also sell an additive to vary the durometer (hardness) rating. I believe McMaster Carr carries it.
http://www.devcon.com/products/products.cfm?brandid=1&catid=3
--
Dennis


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If you don't mix epoxies in the ratio specified, they will not cure properly and excess of one material will slowly degrade. There are a variety of epoxies designed to be rigid and flexible and you should try to find one of these. Also, you can modify somewhat yourself with inert additives. I've used carbon black, sand and fibrous additives to help match intended use.
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