It seems my review of the Sony 100-400mmGM created some questions about how I reached my conclusion regarding the weather sealing.
As stated in the review, I was moving from an area that was cooler with less humidity to one that was warmer and with higher humidity. Naturally, condensation was going to be a problem. To try to counter this somewhat, I placed all of the camera gear I was using: A Canon 1Dx with the 70-200, 200-400 and the Sony A9 and 100-400mm GM lens and a Canon Rebel I was testing with a Tokina 12-24 Lens, in bags. I placed all of them in a bag and gave them about 20 minutes to sit out in the outside temperature as I had some breakfast so that they could acclimate somewhat to the outside temperature. Even so, when I took them out of the bag after about half an hour outside, they all fogged up a bit, with all of the lenses exhibiting condensation on their front elements. The only difference was that the Sony lens showed condensation on both the front and back elements along with some internal condensation inside of the lens. I tried this the second day with the Sony Fe 2x Teleconverter mounted on the Sony 100-400mm GM and a similar issue occurred, just this time the converter also fogged up. The Tokina 12-24 experienced the same, but that is not a weather sealed lens.
I also had an issue with the Sony lens fogging up on my second day of the trip, though in slightly different circumstances. While on the boat during the middle of the day, some heavy rain showers caught us out in the mangroves. Naturally, I placed the lenses and cameras in nylon bags to protect them from the rain (I forgot my rain covers at the hotel) for about five minutes. The issue was that the bags were perfectly sealed and the moisture captured inside created condensation buildup on all the camera gear. This bag quickly picked up a lot of moisture internally and caused condensation build up much in the same manner as had occurred in the mornings. Again, the Sony lens had condensation inside of the lens while the Canon glass did not.
During the mornings, all of the Canon lenses fogged up on their front element but had no issues with internal condensation build up. Naturally, the 200-400 and 70-200, due to their lack of extension will they zoom, are better suited for dealing with such conditions. It took about thirty minutes for the condensation to entirely clear off from the inside on the Sony lens during the mornings and afterward it worked just fine.
I only got the chance to review the lens on one trip and my conclusions are based on only having the lens for a couple of months and one photography trip. The purpose of my review is to give one of but many a user experience and to highlight what I found worked and what I thought didn't work so well. I even stated in my review that many other people are perfectly happy with their copy of this lens when it comes to its handling of moisture. Overall, the Sony 100-400mm GM is a fantastic lens, and I prefer it to its Canon counterpart in most ways, but I was left with more questions than answers regarding its weather sealing.