Wing Planks
The L188 wing is an extreme example of the use of milled skin. The planks were hogged out to on the inside to include integral risers (stiffeners) by giant milling machines and then stretched to removed the warping this induced. I worked Electras at Lockheed New York during Project Uptilt which changed the angle at which the engines were mounted to the wing to reduce the horrendous vibration. Of course, they then required later reworks of the engine mounts and wing due to the whirl mode failures causing loss of several aircraft.
Later I was at EAL at JFK on the Electra phase check line. Lockheed rep told us that they had found milled risers cracking off the planks, in some cases as much as 30-feet - that is feet - these were on some foreign reg aircraft whose owners didn't believe in inspection.
The EAL Electras had doubler patches all over the wings and inside the dry bays the landing gear trusses which were forgings had a lot of repairs as well. Only the small need apply to get into the little access door in the trailing edge.
When I was acting manager at DCA I escorted a Navy P3 squadron commander out to look at an Electra - (the predecessor of his aircraft).
His comment, on seeing all the doubler and tripler repairs on the wings was, "Holy Sh--t, it must have done thirty missions over Hanoi." He couldn't believe all the patches.
At Newark in late 70's, Zantop asked to borrow our hangar to repair an Electra - they were fueling it and had noticed a waterfall coming out from under one wing - one plank had actually broken right across. Completely severed. It took a very large triple-layered repair taking a couple of weeks to engineer and install.
So keep a very close eye on your structure.