Sure it can, works fine. I did it just the other day. It wont do anything if no rust is present other than slightly etch the metal.
You can leave steel bare for several years if treated with phosphoric. However, if a drop of water gets on it it will rust. There are many brands of phosphoric acid. I have been using Ospho for about 20 years after I saw them using it on tug boats and barges at work.
Por-15 is garbage. I have experience with it and have read hundreds of negative posts about it. Many cases where the instructions were followed to a T and the stuff peels off in flakes.
Anyone who is painting over rust is asking for trouble. It may slow things down but rust doesnt sleep.
