I got this response from Hunter:
Thank you for your enquiry.
Occasionally, it is possible that white marks will appear on your Hunter wellington boots. This is a phenomenon called 'blooming' and is your boots attempting to protect themselves from environmental damage.
Every Hunter wellington boot is made from 100% natural ingredients and small amounts of wax are included in the rubber compound during the vulcanisation process that is used to make each pair. When exposed to certain environmental factors, this wax can "bloom" to the surface and appear as white or blue marks.
This is not a defect with the boots, and is completely natural. If this does happen, you can wipe this blooming clean with a warm, damp cloth. Also, Hunter recommend Hunter UV Tech to maintain the ?like new' look of your Hunter Rubber Boots and help to preserve flexibility and colour by providing essential plasticising oils lost by the degrading effects of the sun. Hunter UV Tech can be purchased from various Hunter retailers, as well as from our online store.
Kind regards
(it has to be said, they sit inside by my front door when not in use, and I've only worn them out 3 times, once in sun, twice in rain!)