I use an oil base type paint brush to brush it on and it oxidizes surface rust. Deeper rust takes more time but really beats sanding and grinding away the rust. This is an incredible product.

What's interesting is that you use water to wash away the product and the rust does not reappear once dry. The surface will rust again if you leave the metal out in the rain so you do need to prime & paint before re-exposure to the elements.

This is also handy to use if you have a car that you want to stop the rust but not ready to paint yet. In that case I coat it then leave it on around the bottom of the doors & fenders every 6 months.