The stainless rivets that hold the end cap on have most likely corroded(Galvanic Reaction) with the aluminum making the cap hard to get off.
Why dont they use alum rivets? I just removed stainless rivets from my alum upper rudder castings.... seems like a no-brainer to me?
On items that see cyclical loading (like a rudder arm does), aluminum rivets will loosen pretty quickly. Stainless or monel rivets will hold longer before you need to tighten the joint.
Of all the work I've done on I20s, I've never had to get into the outhaul. I HAVE had them stick and my F18 got quite bad after a capsize left the boom in some sand (long story). To free it up, I usually put the boom in the (freshwater) lake, and work the outhaul repeatedly underwater. It seems to have fixed every sticky one I've had to deal with.
As far as getting the endcap off, I would recommend the use of a body repair slide hammer. Drill a hole in the end of the cap (or a few) and work at it with the slide hammer. If that won't get it off, you should prepare to cut it out.
