A few years ago I was mapping a route and I saw it went over something called "sand mountain" I didn't think much of it until I got there. It started with a little sinking here and there and soon progressed to full blown walking for miles. It was a point to point ride so I needed to get through it, I couldn't turn around. It was awful.

That was on a gravel bike with 38's. The same bike currently has 43's and I wouldn't go back. My mountain bike has 2.5" tires and I wouldn't go there either. It's too steep and sandy to really ride a fat bike, it might not sink, but I doubt it would be rideable. The only thing I'd consider for a return trip would be a fat tired e bike or an ATV. Interestingly, I hear sand mountain is lovely in the spring for a week or two after the snow melts.

For truly sandy riding, my suggestion is to not do it. Just skip the entire thing and find somewhere else. I'd rather take a spinning class at the rec center than ride that ****.

If it's just a patch or two, low gears and as much weight on the back as possible helps. Don't stand up. If you can, try to hit it after a rain. The sandy sections of the White Rim trail are very doable if a little unnerving on a gravel bike using that technique.