46% humidity is too low, you need at least 80% and that is for a 3-4 hour rust. At your low percentage, it will have to be at least 6-8 hours or more, maybe overnight, but be careful in not putting too much rusting agent on them or it could pit.
In not having a trough long enough could be another reason and I'm sure is the reason your barrels are not coming out. Instead of being a 5-7 second etch they are know quite a bit longer as the etchant is still working on the part that is not submerged.
If there is a Tractor Supply near you, they sell plastic food/water troughs that hand on fencing.
That is why I prefer to rust in the spring/summer, I use a outside faucet with a hose running and when I take the barrels out of the etchant, they go right under the hose, and then I card with heavy denim.
