Continued black toenails led to efforts to compress the toenail area.  This has led to subjective improvement.  The latest theory is this area not compressing while the rest compresses, leading to pressure on the toenail area.  The real need is a way to taper the EVA thickness.  The area being clamped needs to be 10mm or less.  The ball needs to be 16mm.  The taper needs to be gradual enough to not create a chafe point.

Heat can burn off such a gradual taper, in exchange for a lot of fumes.  The gootube shows box cutters & xactos being used to create large angle cuts.   Hot wire seems to have fallen out of favor in the last 20 years.

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After much clamping of foam, a 16 miler in hot weather created pretty blistered toes again.  The clamping didn't really compress it, but theories still circled back to fake leather in the toe area & the age of the fake leather making it more abrasive.  As the plastic coating wears off, the toes are sitting on underlying fabric which is getting soaked & more abrasive.  Fake leather also leaves a trail of microplastics.  It's horrible stuff in every way except grip in rain.

Lions previously used real suede.  As noted previously

https://hackaday.io/project/88623/log/225066-luna-update

https://hackaday.io/project/88623/log/220493-pleather-failure

real suede shrinks.  The 1st experiments with pleather arrived at needing vaseline for the toes.

The next idea was finally going with bare EVA in the toe area, with an abrupt transition.  The worn out areas of pleather were proving immune enough from blisters to believe the transition would also be immune.  Taking pleather off requires taking a layer of EVA off, which makes it more abrasive.  Tests with the bare EVA region showed promise, but the exact shape of the transition would require some experiments.  New sandals should probably hot snot the pleather so it can be either replaced often or reshaped.

The ideal toe area would have PTFE.  To have a PTFE toe area, any PTFE sheet would have to be sewed on.  No adhesive would keep it on.

Another idea was replacing 6mm of EVA with real leather.  

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The bare EVA section in the toe area was encouraging, after several weeks.  Distances up to 18 miles didn't cause any more toe blisters of the previous scale.  In rain, enough pleather was left to provide sufficient grip.  The toe area doesn't have any effect in rain. The optimum cut seems to be a straight line.  This leaves enough pleather for rain.