I have done the multiple day soak in plain old white vinegar. Works great. As far as agitating it, I used a piece of garden hose and connected it to my compressor hose. You can direct the hose around inside the tank. I also take the petcock off and blow some air into the petcock holes. It cleans out the tubes that run into the main and reserve feeds. Never have the tank cap on while doing this. The tank can only take a few psi of pressure. To clean it out, I use the same garden hose and duct tape it to a shopvac hose. It is much easier to get all of the cleaning contents and gunk out when the tank is out of the bike.The chances are that what you are seeing in your tank is NOT RUST! Almost for sure its corn syrup. It's visually indistinguishable from rust. Fill your tank up with some parts cleaner and let it set over night, or better yet out side on a hot sunny day with the seat and side covers off so the sun can heat it. Then siphon the parts cleaner out. You will be amazed. If you can figure out a way to agitate the parts cleaner that would be even better. I've cleaned maybe 30 tanks this way,it works.