This is the thread that has inspired me to get back into some of my projects. The detail is simply amazing to witness.
Backing up a few posts, I've also used the Krud Kutter mentioned, both spray and brush on. It works, but usually needs more than one application and best when brushed on, let dry and turn white and brush again. Then it turns black.
I've had better results with stuff called Rust Kutter, from Tractor Supply. I think both products are basically phosphoric acid, but this one stays wet a very long time. I believe that's due to some surfactant that is added. I use good old WalMart great value dish detergent added to sprays I use in our farm pastures as a surfactant. Does a mean job with less chemical stuff needed in the sprayer.
The 128sl gas tank behind the.spray bottle was covered in surface rust before spraying about 4 hours ago.