Editing Phosphoric acid as metal pretreatment

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The use of phosphoric acid-based products as metal pretreatments is controversial in hotrodding. Proponents favor its low cost and ease of use, while opponents cite the problem of acid film leading to topcoat failure.

Phosphoric acid converts [[rust]] to black ferric phosphate.

==Brands of phosphoric acid-based metal pretreatments used for automotive work==
*[http://ospho.com Ospho] (75% phosphoric acid)

==Naval jelly==
Naval jelly is considered safer to use than traditional liquid phosphoric acid metal pretreatments. For rust removal on body panels, the naval jelly is kept wet. Paint failures from phosphoric acid are thought to be caused by wiping on the acid, and letting it dry.

''need details here on exact concentrations, and use of naval jelly vs. traditional phosphoric acid treatments''

==Reasons for using phosphoric acid==
*Inexpensive.
*[[Media blasting]] has its own set of drawbacks.

==Reasons for not using phosphoric acid==
*Metal pretreatment manufacturers are economically incentivized to recommend their products. A better point of view might be obtained from the manufacturer of the product that is destined to go on top of the pretreatment.

*Even a very low failure rate is economically disadvantageous, especially in light of the value of a professional paint job, and the labor required to fix a failure.

<blockquote>
''"Even though I believe I can use these products correctly I will not take such a risk when there are alternatives. It will only take 1 or 2 to put you out of business or in court."'' -- '''shine''', in [http://www.hotrodders.com/forum/ospho-factory-response-173513.html#post1234352 this post on the Hotrodders Bulletin Board].
</blockquote>

*A manufacturer's tech sheet may specifically advise against using an acid pretreatment. Always follow the tech sheets for the products you are using.

*Amateurs and [[First paint job|first-time painters]] are more likely to make errors. Phosphoric acid pretreatments must be used meticulously, and are thus not recommended for beginners.

==Specific paint and primer failures caused by phosphoric acid pretreatments==
*Bubbles of paint lifting, especially at seams.
*Epoxy cracking.

==Published epoxy primer and paint manufacturers' positions on the use of phosphoric acid pretreatments==
===Southern Polyurethanes, Inc.===
*[http://www.southernpolyurethanes.com/tech%20sheets%20new/epOXY%202006.pdf Southern Polyurethanes]:
<blockquote>
''"NEVER use SPI Epoxy over a Soda Blasted vehicle, Acid Etch/Wash Primer, Rust Converter or other Metal Treatments. NEVER!"''
</blockquote>

The owner of Southern Polyurethanes has detailed some options that '''should''' solve the phosphoric acid film problem, in [http://www.hotrodders.com/forum/ospho-factory-response-173513.html#post1235021 this post on the Hotrodders Bulletin Board].

==Use of phosphoric acid in an industrial setting==



==References==
*http://www.hotrodders.com/forum/ospho-factory-response-173513.html
*http://www.ospho.com/directions.htm
*http://www.ospho.com/images/2008%20MSDS.pdf
*http://en.wikipedia.org/wiki/Phosphoric_acid