About 35 years ago I bought a project car in Gallup NM on the Navajo Nation, and someone had done some sanding on the body and left it outside with bare metal in spots. I was told that had been done quite some time back, but there was hardly any rust. Of course that doesn't happen in very many places because of acid rain and the fall out from the industrialized world we live in.
Phosphoric is on the mild side for acids, so it can be diluted to the point of being harmless and just rinsed off even with the tap water we have, but strong acids are a different story. Everyone knew that in the 1950s and 60s, when our car batteries were vented and had removable caps, even the fumes from strong acids will cause rust. So occasionally we would mix some baking soda and water to dribble on the battery and its tray to neutralize the acid, no one used a fire hose to get rid of it. It takes a strong base to neutralize a strong acid.
Here's something that I've been doing lately with phosphoric acid and it has worked well for me. Of course acid works by dissolving the rust, so the more the rust is scratched the more area to dissolve. After the rust is gone, wet the whole area good with acid and scrub with a wet red scotch bright pad. If working on the whole panel you may need a helper to keep it all wet while scrubbing, that gets it all ready for rinsing. Spray it all wet with acid when done scrubbing, then switch spray bottles to Zep Industrial Purple Degreaser and start at the top of the panel and keep spraying all across the top until you have rinsed all the acid down the panel. It takes a lot of spraying and your fingers will get tired, but this procedure will get rid of it, you just have to be thorough. No need to worry about flash rust, that is one reason for using the Zep degreaser, it has a ph of about 12-13, so it will be of considerable help to inhibit rust. Its not needed to neutralize the acid, its just a safe way to keep the panel wet long enough to rinse thoroughly. When the acid is all gone, then rinse with water. Zep degreaser is available at home improvement stores for about $10-$15 for a concentrated gallon, that makes 25 gallons of usable degreaser.
This whole process requires the use of eye and lung protection when spraying either acid or degreaser.
Keeping the acid wet, using the scotch pad, and thorough rinsing are all key to getting a clean acid free panel. When you are finished, the panel will be clean and shiny and will be protected from rust for a while.
