I use the Devcon 5-Minute Epoxy and yes one needs to mix the amounts of both the resin and hardener as close to 1:1 you can or it probably won't set up and dry the way it should. As far as shelf life, if you don't open the packages and puncture the tubes, it should be good for a long time as I never have a problem with fresh packages of epoxy lying around for months. I've been using epoxy as my go-to glue for many years now and I like it because it has a strong bond and dries clear. I never have to take a "repair kit" when I bring my models to display at club meetings, NNL's, or shows as nothing falls off.