Take a wire brush to the wiper shafts, the smooth surface you see may be white metal ground off on the splines, they are quite fine. Try Locktite instead of epoxy.

I had the same problem with our '88 745 about 100,000 miles ago. My routine for wiper arm stripping -
1. Add a washer, this pushes the arm down further onto the tapered shaft.
2. Wrap some fine copper wire around the shaft - my alternative to tin foil - this adds material to shaft, making a tighter fit.
3. Get a new wiper arm.

Clean the shaft before any repairs. Be sure to free the arms up if they have ice on them, being stuck aggravates any slipping issues. When driving in snow, clean off snow build-up often for the same reason.

At 230,000 miles, still using the same shafts.
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3 8s & 2 7s 725,000 miles total