If you use it on bare 'clean' metal you need to use the marine clean and metal ready (I think its a zinc coating or something) and it will stick fine. If it is used on bare metal without the proper prep it will not stick well at all. On rust you just need to remove the scaly loose stuff. Also, if you use the flatter black, its inconsistant and you may end up with shiny and duller spots. I painted my entire cage with it after cleaning and prepping and it stuck great. After masking the whole cage off I had one tiny piece flake off. I must have missed a spot. Someone else painted over some paint around my back window and half of all the paint in that area peeled offf on the tape.
I would take the time to read the directions and you will be fine. If your not sure call them. I did.
