This time, I spread my wipe-on protectant ('Skin-Prep') on a taller and longer area, allowing for a strip of Flexifix to cover the most of the "upper" half of the Sensor's adhesive pad. This is not complete coverage; The Transmitter and bottom half of the adhesive pad are still exposed to open air. But across the top half, I've got a horizontal strip of Smith & Nephew "FlexiFix". It's 4" long (the width of the 'wide' roll which I bought), and it's about 1" tall.
That size provides about 1/2" of skin contact at the tallest part of the Dexcom oval, and increases skin coverage as the Dexcom pad gets less tall (towards the edges). My Flexifix strip length is the entire 4" width of the "wide" package, so that leaves about 1/2" extra at the ends (sideways), where it's completely on skin. I'm now on day 10, which is about when I'd begin to see tiny bits of black muck at the edges of the Dexcom adhesive pad. (With Skin-Prep applied underneath, but nothing else). Often, during days 10-14, I've needed to "patch up" the edges of my Sensor adhesive pads....
And Mastisol, once used, requires a strong chemical "remover" to take it off. Also, it isn't (for me) a permanent fix, and often needs a re-application a few days later. So for me, this looks like better performance, quicker to do, and easier on my skin, and permanent: A total winner.
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BTW, it's not due to less physical activity while wearing this particular Sensor: We just had a Class-2 Hurricane in my neighborhood (wind and rain, but inland, without "storm surge" tides). I'm wrassling several of busted-up fence posts, along with all the rails, planks, and battens which had been mounted on them. :((