RE: "Defective" Epoxy?
Hardening time has a lot to do with temp, and humidity. I just used some Epoxy 220 to glue the wings together on an ARF I'm building, and the next day the epoxy that was under the masking tape was sitll sticky on the surface. I've used this epoxy for years and sometimes it comes up rock hard in just a few hours and then other times it will be like the wing joint. It does finally harden though.
A 100 watt lamp in a desk lamp setup around 6" away from the joint will provide enough heat to fully harden the Epoxy 220 in just a couple hours. I've heard of guys putting their whole fuselage in a car with the windows rolled up in the sun to accelerate the hardening. Don't use to much heat though. around 130F is just about right.
Don