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, especially when exposed to UV radiation. Similar issues surround the use of [[soda blasting]].

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

==Brands of phosphoric acid-based metal pretreatments used for automotive work==
===Ospho===
Ospho is advertised as "a balanced formula of Phosphoric acid, Sodium Dichromate, Surfactants, and Extenders." According to Ospho's manufacturers, as detailed in [http://www.hotrodders.com/forum/ospho-factory-response-173513.html#post1234300 this post]:

#Once Ospho has dried, there is no longer any acid on the surface to react with paint.
#Ospho is totally neutralized by reaction with both the rust particles and the steel underneath. A quick water rinse will remove any dust-like reaction particles.
#Because Ospho is water-based, the water can hide in nooks and crannies, causing adhesion failure of primer.

==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.
*If used perfectly, it works perfectly.

==Reasons for not using phosphoric acid==
*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 may be beneficial in an industrial or manufacturing environment. However, they can be catastrophic for beginners.

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

==Published positions from primer and paint manufacturers==
===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, while confirming that he doesn't promote the use of phosphoric acid, 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
*http://www.skybryte.com/index.html
*http://www.ospho.com/data.htm

[[Category:Body and exterior]]