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 is advertised as "a balanced formula of Phosphoric acid, Sodium Dichromate, Surfactants, and Extenders." According to Ospho's manufacturers, as detailed in 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.
- Because Ospho is water-based, the water can hide in nooks and crannies, causing adhesion failure of primer
For details on Ospho use, see Rust removal with Ospho.
 Application tips
- A quick, light water rinse with a damp towel or a wax and grease remover will remove any dust-like reaction particles.
- Always make sure metal is dry before using ospho or priming.
- Sand or scuff before priming then use a wax and grease remover to clean.
 PPG DX579
PPG DX579 contains a lower concentration of phosphoric acid than Ospho.
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.
 Reasons for using phosphoric acid
- Media blasting has its own set of drawbacks, especially when done by a hobbyist or amateur: panel warpage, health hazards for certain media, and mess.
- If used perfectly, it works fine with a variety of epoxy primers.
- No experience is needed; easy for amateurs to use.
- Protects bare metal from surface rust.
 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.
"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 this post on the Hotrodders Bulletin Board.
- 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-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
 House of Kolor
"NOTE: Do not use any acid base products such as self etching primer, etc. under the KP2CF primer. This will almost certainly cause an adhesion problem.
NOTE: If you find it necessary to use a metal conditioner to remove rust, etc., be sure to thoroughly clean and neutralize the treated area following the conditioner manufacturers recommendations, then using our KC20 post sanding cleaner with a maroon scuff pad to insure all acid residue has been removed before priming. If not, this will almost certainly cause an adhesion problem."
House of Kolor is a division of Valspar Corporation.
 Southern Polyurethanes, Inc.
"NEVER use SPI Epoxy over a Soda Blasted vehicle, Acid Etch/Wash Primer, Rust Converter or other Metal Treatments. NEVER!"
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 this post on the Hotrodders Bulletin Board.
 Use of phosphoric acid in an industrial setting
this section needs development
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