I use Phosphoric Acid for cleaning metal all the Time when I need too after blasting and or Wire wheeling for Heavy pitting and heavy Rust cleaning I start with Kleen strip from Home Depot its pretty strong and removes heavy rust BUT I also go farther with any metal cleaned with Phosphoric acid which works well for me.

I use Duponts 5717s followed by Duponts 5718s they are now made by Axalta under the Cromax Banner. They were originally designed to be used together and in my opinion I will always use them together one after the other and washed thoroughly with soap and water after words.

The 5717s is a Phosphoric acid wash cleaner that cleans and converts light rust (Iron Oxide) into "Black Iron Phosphate" generally this is after I have either Blasted my metal clean or used a Good hi quality wire wheel on it or both removing as much rust as possible.

5717s is applied liberally and kept wet! I scrub with Scotchbrite pad After rinsing toughly the clean rust free metal will flash rust ever so lightly as it drys don't be alarmed or worried this is the result of clean grease and oil free metal being exposed to oxygen its normal and it will flash rust! Now you could scrub the Flash rust dust off with a Scotchbrite pad and a clean towel and just paint it BUT the smart thing to do when its dry is it Hit it with the 5718s as it was designed to be used with.
http://m.products.axaltacs.com/mcat/us/ ... obile=true

The 5718s is also applied liberally and kept wet it Etches the metal cleaning converting that light flash rust instantly when applied and deposits a coating of ZINC Phosphate!!!!!! to the metals surface, Perfect for Painting and Long term Rust protection!
This product must be Thoroughly washed with Soapy water before Painting!
http://products.axaltacs.com/dcat/us/en ... 5718S.html

These Products are the only ones designed to be used under a complete Paint system that I know of? Where a Paint mfg will stand behind a rust converter using them in a complete system together.

Instructions AND Precautions must be followed! to the letter with no deviation! This can not be stressed enough! This is a professional product Not some junk your gonna get at your local home Depot or auto parts store It deserves respect follow the instructions!
I have treated all my bare metal Full repaint jobs with these products with no Problems.
I have contacted Dupont for more info on product and its use before Axalta took over, Stumping the Tech guys who had to go back to engineers for help answering my questions. For me this is the Best and the Only Product that I will use on any of my cars metal for removing rust before paint. To me what could be better than a coating of Zinc followed by Two coats of Epoxy?
I have been playing around with the idea of using an etching primer instead of Epoxy to keep my costs down but haven't done it yet.