One of the pieces of gear on the UTMB mandatory equipment list is a pair of waterproof gloves. Fine in theory, the only problem being that fully waterproof gloves for running practically don't exist. Sure, there are some such as Sealskins or Raidlight mitts but these seem over kill for the task at hand. Others just have a waterproof topside or fold out mitt, but neither of these would really qualify as fully waterproof. The other option is clearly to move towards winter mountain/skiing gloves but clearly these are hardly ideal for carrying around Mt Blanc in a pack. Some people get around the problem by carrying latex surgical gloves but these are practically useless if you ever need to use them, with stories abound from previous years of peoples gloves filling up like water ballons.
After a bit of research I identified a few of potential options but wasnt convinced by any of them so I decided to make my own.
I had some very lightweight Cuben fibre non-woven PE laminate lying around from a previous project. If you are not familiar with Cuben, it is a crazy material originally developed for americas cup racing sails. It has since been used for many different applications and has been adopted by the ultralight backpacking/ make your own gear crowd as it is extremely strong, completely waterproof and incredibly lightweight. It also isn't cheap.
One of the other advantages of Cuben is that it likes to be bonded, as opposed to sewn, which means that it is possible to create very strong waterproof seams without the need for stitching and taping as is common with most waterproof clothing construction.
In the end I decided to make some simple over-mitts with an extended cuff and a simple elasticated draw string which I could pull over my thin running gloves if needed. The longer cuff could then be tucked deep into the sleeve of my rain jacket to ensure that there would be no risk of rain getting in. Construction wise the individual panels are bonded using a permanent waterproof 3M spray adhesive to create a folded overlapping seam. The channel for the drawstring is created using a thin strip of specialised Mylar double sided tape. The drawstring itself is made form a very thin elastic cord with a heat shrink gripper tab on the end. It can be used and released with one hand.
The end result is a one-off pair of fully waterproof mitts which weigh 8g for the pair and pack down to virtually nothing.
Of course in the end I didn't need to use them.
