I usually let it soak for 24 hours. Make sure no EvapoRust gets on any bluing. If you are just cleaning the bore and don’t want to harm the bluing, you need to find a way to hold the barrel vertical while filling it and to hold it steady while EvapoRust does it’s thing. I held the barrel in this photo with a woodworking vice mounted to the side of a bench before careful filling the bore to the top. One trick that I learned was to fill the barrel with water as practice run, then pour the water into small clear container. Mark the container on outside to show the amount of water held in the barrel. You then have a measuring device for the exact amount of EvapoRust you will need. Avoids mess of overfilling bore.I’m getting ready to use Evapo-Rust for the first time on a mildly rusted bore. How long should I let it remain in the barrel?