Not aimed specifically at keeping toes clean, but putting copious woodchips down on all paths makes a big difference for spread of mud. I can walk our garden in winter in bare feet and return to the house with my feet essentially clean. Big difference from a few years back when our paths were treacherous mud for much of the year.
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Years ago when I did a lot of mechanical work at home, and those thin neoprene mechanics gloves didn't exist, I used a hand cream product that was applied before starting work. It allowed good grip but nothing would stick to it, including enamel paint.
So, similarly, perhaps you could apply a good layer of, say, lanolin cream to your feet and around the nails to make cleaning them a bit easier.
Obviously, wearing long trousers over the boots would stop crap falling into them too.
Alternatively, you could make a pair of short gaiters out of cloth and elastic and drape them over the top of the Wellies - a bigger version of those in the photo.
'Every time I learn something new, it pushes some old stuff out of my brain.'
i have heavy, clinging clay mud. I paved my main path (which goes up a steep slope) and the rest of the paths between the beds (which are walled in and theoretically not getting trodden on) are covered with broken crockery, coconut shells, rocks i've taken out of hte beds, etc, which helped a lot. my feet are usually okay but my hands are exactly like you describe, no matter what I do i've always got mud under my nails.
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