Originally Posted by Spraytek
I've read on a few different sites that oxalis acid would work, but that it was extremely toxic. I'm assuming they meant the fumes.
No, the chemical. Don't eat it, and wear gloves. It isn't really that bad.
Oxalic acid is the toxin in rhubarb. The "fumes" are just fine.
Muriatic acid fumes are VERY bad, but muriatic acid will also etch stone pretty horribly. That's not necessarily a problem on concrete where it won't be noticed, but it is a problem for you.
Vinegar is about as acidic as oxalic acid, but oxalic acid is stronger at removing rust (and oils as Ric discovered on asphalt) than vinegar. Vinegar should work as would CLR/Limeaway, which is phosphoric acid, but would be slower.