It looked kind of old. Hopefully it still works. I will find out! It was free and I can't argue with that!
I put it on the bumper seam and the rear body seam.
It turns the rust a dark color. It says that it reacts with the rust and totally seals it, stopping it from progressing. Sometime next week I'll paint over it with some primer. If it doesn't screw the primer up I think I'll be in good shape.
I also removed the right hand rear quarter window and weather stripping to see what was underneath.
It looks pretty good. Just a really small spot of rust. It will get sanded and treated with some more rust-proofer.