Not sure what paint type you have on your vehicle, but many black paint finishes today are as well clear coated.
While it might be difficult to see Iron-X working on some dark colors, when it comes time to rinse the product off, and you see lots of purple color all over on the ground, you will then know it has done its job.
Just follow directions, and the most important is to not let it dry upon the paint. Working in the shade on cool paint and with as cool of temps as regionally possible will be preferable. You must work quickly to cover the entire vehicle, then wait for the product to do its job.
One does not have to coat every square inch of paint work with the spray. Doing it that way, you will go through a lot of product unnecessarily. Usually most use a few spritzes or so per panel, then spread with a slightly wetted sponge. Then move onto the next panel applying.
Have a clean bucket of water on hand as you go, so you can slightly re-wet the sponge as needed.
Hope this helps.