LOL... I'm sorry for this, but had to take a jab... Every wing I've ever put together always says to use 30-minute epoxy for gluing the joiner and joining... Theres a reason.. Anyway, heat gun won't work, you'll never get all the epoxy soft everywhere along the joiner... You best bet is to do as has been suggested... Cut it in half and redo it... What you'll need to do is first get it in half... Then use a roto tool to get out as much of the old joiner as you can.... Make a new joiner that fits and do it again... This time you'll have to measure the dihiedral yourself and make sure it's right. Also, when your done, since the new joiner is not going to be as strong as the old, you need to re-enforce the wing... Go back to the hobby store (while your picking up the 30-minute epoxy... and get about a 1 1/2" or 2" wide strip of fiberglass cloth... When the wing is done, mix up new epoxy... Take a brush and brush the epoxy on in a very thin coat over the cloth (the cloth will wrap all the way around the wing at the joint..) When your done you'll have a fiberglass re-inforced joint that won't go anywhere...

Course, as has already been mentioned.... If this ARF is from the Empire, they probably sell a new wing for it... It would certainly save a WHOLE lotta hassle... If it's not from the Empire, you may be able to contact the company directly and see if you can get a new wing from them....