As noted above, there's only so much rust it can absorb before it's used up. It's better to remove all the loose rust you can beforehand. When it's not yellow any more, you can't bring it back.
It's not a solvent, so you also need to clean off all dirt, oil, wax, grease, etc. beforehand.
Most other rust removers are acids that will also attack the metal; Evapo-Rust only attacks rust, so it's quite safe to leave the item soaking for as long as needed. I've left gas tanks sitting for months until I had time to return to the project. With really rusty stuff, take it out, rinse and scrub to allow the solution better access to the remaining rust, and soak again.
It won't directly attack chrome, powder coat, paint, rubber, plastic parts, but if there's rust underneath it will usually take out the rust, so coatings or parts can get loose.
On the bottle, it tells you that it also can protect metal; you can brush or spray it on and let it dry.
It's also important to understand that rust is a combination of some of the iron from the object and oxygen from the air. This has two effects:
1) Rust takes up more space than the iron did, a phenomenon called rust jacking. This can actually bend or compress mating parts, so when you remove the rust, the fit may be uneven or loose. For example, the threads on rusty fasteners might become un-usable, or through-holes might get warped.
2) When you remove the rust, you remove the iron that was in the rust. Seems obvious, but a lot of people don't get that and have complained that Evapo-Rust left their item pitted or with a rough finish, or parts or finishes get loose. It's not the Evapo-Rust's fault!
I really like the fact that it doesn't have an odor, it's safe on skin, it's not acidic, and it's not toxic. You can dump it down the drain when it's used up, or if your soil is iron-poor, folks have even reported putting it on their flower beds with good results.
