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#529277 - Fri Apr 17 2009 03:22 AM Phosphoric acid neutralizing  
Dan B.  Offline
Wrench Fetcher
Joined: Mar 2009
Posts: 72
I have been using phosphoric acid to treat most of the paintable surfaces on my 3100. Just ordered epoxy primer and am having second thoughts about shooting over the acid prepped metal. I read the whole thread about "phosphoric acid rules" but not much mentioned about whether it needs to be neutralized with anything before priming. This first step is obviously critical. Anyone with long term experience with this stuff?


#529296 - Fri Apr 17 2009 03:45 AM Re: Phosphoric acid neutralizing [Re: Dan B.]  
Flxible  Offline
Joined: Feb 2002
Posts: 16,453
north of the 49th
I haven't found it cause any problems, it's great metal prep for rust prevention, but also best practice when starting to shoot paint is to wipe down with a grease/wax remover immediately beforehand


"When we tug a single thing in nature, we find it attached to the rest of the world" ~ John Muir
"When we tug a single thing on an old truck, we find it falls off" ~ me
Some TF series details & TF heater pics & Rust-a-holics Unanimous parking lot

#529321 - Fri Apr 17 2009 04:44 AM Re: Phosphoric acid neutralizing [Re: Flxible]  
Slickriffs  Offline
Shop Shark
Joined: Jan 2007
Posts: 748
Fairfield CT
Yup-pretty standard series of events here-Never had a problem.


#529398 - Fri Apr 17 2009 02:09 PM Re: Phosphoric acid neutralizing [Re: Slickriffs]  
Tiny  Offline
Global Mod
Joined: Aug 2005
Posts: 10,909
South Central Kansas
If neutralizing it will make you feel better a simple baking soda/water solution will kill any residue.

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#529405 - Fri Apr 17 2009 02:35 PM Re: Phosphoric acid neutralizing [Re: Tiny]  
sclor  Offline
Wrench Fetcher
Joined: Jan 2007
Posts: 104
New Orleans
I use Ospho in a spray bottle, scuff with a scuff pad, then wipe with a paper towel. When I am ready to prime, I scuff with a scuff pad, then wipe with Wax and Grease remover. I have never had any problems.

The one caveat is that you can only apply phospohric acid once. Thus you have to be sure that the primer you are using doesn't also contain the acid. That means no etch primer and no hybrid epoxies that contain acid. I use Kirker Enduro that doesn't. I believe Valspar/House of Kolor is one that does contain acid. It is best to look at the manufacturers data sheets for the primer you are using to ensure that metal etched with phosphoric acid is a suitable substrate for the primer.

I hope this is helpful.
New Orleans

#529457 - Fri Apr 17 2009 07:19 PM Re: Phosphoric acid neutralizing [Re: sclor]  
Spotbiltxo  Offline
Shop Shark
Joined: Jun 2008
Posts: 913
San Antonio, Texas, USA
Steve makes a good point, but I wouldn't expect acid-acid reaction with a previously treated surface that is totally dry.
Once the phosphoric acid reacts with the rust to form oxide then the acid has been altered. Significant reaction would occur if the organic acids are viscueous.

As a note, it is always prudent to neutralize any surface treated with acid with fresh water. Most of the directions on a bottle of phosphoric acid reccomend this step in their directions section.


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1951 Chevy 1/2 ton
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#529507 - Fri Apr 17 2009 11:00 PM Re: Phosphoric acid neutralizing [Re: Spotbiltxo]  
Jimmie D  Offline
Shop Shark
Joined: Feb 2006
Posts: 548
Grand Junction, Colo.
Here is what Wikipedia had on the subject.
Rust removal
Phosphoric acid may be used by direct application to rusted iron, steel tools, or surfaces to convert iron(III) oxide (rust) to a water-soluble phosphate compound. It is usually available as a greenish liquid, suitable for dipping (acid bath), but is more generally used as a component in a gel, commonly called naval jelly. As a thick gel, it may be applied to sloping, vertical, or even overhead surfaces. Care must be taken to avoid acid burns of the skin and especially the eyes, but the residue is easily diluted with water. When sufficiently diluted, it can even be nutritious to plant life, containing the essential nutrients phosphorus and iron. It is sometimes sold under other names, such as "rust remover" or "rust killer." It should not be directly introduced into surface water such as creeks or into drains, however. After treatment, the reddish-brown iron oxide will be converted to a black iron phosphate compound coating that may be scrubbed off. Multiple applications of phosphoric acid may be required to remove all rust. The resultant black compound can provide further corrosion resistance (such protection is somewhat provided by the superficially similar Parkerizing and blued electrochemical conversion coating processes). After application and removal of rust using phosphoric acid compounds, the metal should be oiled (if to be used bare, as in a tool) or appropriately painted, by using a multiple coat process of primer, intermediate, and finish coats.

#530138 - Mon Apr 20 2009 12:22 AM Re: Phosphoric acid neutralizing [Re: Jimmie D]  
50adrod  Offline
Wrench Fetcher
Joined: Dec 2007
Posts: 196
I use SPI epoxy and they advise against it. they recomend sanded bare metal only. if you are using SPI epoxy just give Barry a call, he will help you out.

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