On three occasions now I have mixed up a small batch of acra-glas to repair a small forend crack. I put a dab of brown coloring in the mix to match the wood. What I get is like a hard putty; it just does not cure. What am I doing wrong? I use the 4:1 ratio of epoxy to hardener (roughly) and just a tiny bit of coloring. I've had the stuff a couple years now, but the box says it's good for 7-10. Thanks