Phosphoric Acid Metal Cleaner
There is a plethora of metal cleaning products available in industry as well as to the average consumer. Many metal projects involving welding,brazing, and painting demand a metal surface free of oxidation and general contamination. Degreasers remove lubricating oils and grease but not rust and tarnish. Most rust removers are acid based cleaners or chelating agents. Acid solutions are corrosive and toxic but they act rapidly on rust and tarnish. Muriatic acid, available at many home improvement centers, is the most active but leaves metals in an activated state that makes them rust even faster and more extensively. The acid in the featured product is a 25 to 40% phosphoric acid solution. Phosphoric dissolves rust a little slower but leaves the metal with a rust resistant finish with iron phosphate that is ideal for priming and painting. 
Steel Soaking in Phosphoric Acid
In the photo above, is 18 gauge cold rolled with surface rust soaking in the prep and etch. Aside from the phosphoric acid, this cleaner has a surfactant to aid in wetting.
The Next Morning
After leaving the steel in the phosphoric acid overnight, this is what I found. The metal had a more consistent finish and was overall grey. This is iron phosphate. This would be ideal for priming and painting. For welding, I would wire brush this to make sure no phosphorus vapors or formed. Also, phosphorus in element form, may make steel excessively brittle.
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