My tank was very rusty (dad said it had stalagtights (sp)), but used the molasses first, left it in for about 12 hrs I think, had some result, but wasn't fast enough, so next I put some "the works" and water in, which was faster, but didn't leave it long enough. Fast forward about 2 months, and I did the works again, about 4:1 4 parts water, filled it to the top and monitored it for about 20 hrs. I left the tank full enough to take the rust off the bottom of the cap. I think it will have ill effects on your petcock, so watch out. It's also hard on paint (took about a 1/4" ring around the tank mouth and softened it , so I just pulled it off. I think it worked it's way under the paint there) Also, watch out for flash rust. But I now have a nice and clean tank.