Thin brass shim stock will work better than foil. It is more substantial but is harder to come-by.
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Tom 69-1800S, 72-1800ES, 96-850T

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

I know this may sound stupid, but I'd try it. Take a magnifying glass and look at the shaft closely. Get a hold of a dental pick and see if you can "scribe" lines, meaning, you might be able to see the start of an existing line and scribe it further down hte shaft. Call me kooky, but, I've done this tirck on splined shafts from old car radios back in the 70's.

Good luck

never had the problem but i would consider JW-weld.which i think is a form of epoxy.

The wiper arm doesn't have splines, a new one will be perfectly smooth. It's made of a relative soft metal that gets indentations from the splines on the shaft. If it will tighten down and not wobble or slip, nothing else is needed. If not, a temporary fix can be made by wrapping tin foil (or anything similar) around the splines on the shaft, then force the arm on and tighten the nut. A permanent fix will require a new arm. These are about $50.00 retail.