I’m not the best person to explain pre-soaking, because I’ve been fortunate to not require it. But it definitely sounds like you’re a good candidate for it.
You basically insert a new sensor early, and completely ignore it for up to a full day. Don’t stop the existing sensor session, don’t insert the transmitter… just ignore it. How long you need to “pre-soak” the sensor depends on how long you usually get crazy, erratic readings when you insert a new one. 6 hours, overnight, 12 hours, 24 hours?
There’s some trauma at the insertion site, along with inflammation, and immune response. That’s the cause of the first day sensor blues. You’re basically just giving it some time to heal from that trauma before you actually plug in the transmitter and start collecting data from that new sensor.
And trust me, you have my sympathy about the rashes. I’ve been right there with you for the last six months or so. Dexcom made a TERRIBLE adhesive change earlier in the year that a LOT of people were allergic to. They had to change back because of the complaints, and I think I’m finally getting back to non-itchy sensors. For the first time in forever with my most recent I haven’t been trying to claw a sensor off.
Have you tried the Flonase (yes, the nasal spray) trick? It doesn’t work for everyone, but I’ve had really good luck with it. You spray it all over the area you’re going to apply the sensor and let it dry, then apply the sensor on top of the Flonase. If it starts to get itchy again, I get a lot of relief by soaking the sensor tape with it. I buy the generic equivalent from Sam’s Club, too. It’s pennies on the dollar, compared to the brand name stuff.