On last Sunday, I got tired of looking at my door rust and took a wire brush on my grinder and ground as much of the rust from the bottom of the door, inside and out. I then sprayed it with 4 coats of this permatex product. It turns any rust to black and then it can be sanded, primed and painted. After completely all of that and also repainting the inside of my door sills, I am so glad to see no rust spots. It feels like I am no longer driving a rust bucket. It may be a cheap fix, but definetely better than buying new doors.