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Phosphoric acid vs. Muriatic acid?

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01-12-2002 07:43:28

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I noticed the postings below about phosphoric acid. I was wondering when/if to use phosphoric or muriatic acid to clean rusty parts. I've used the muriatic acid before, but is the phosphoric acid used for a different purpose or is it superior to the muriatic acid? Clooney, you mention "The Shop". What does their phosphoric acid cost? Overall, is the phosphoric acid more expensive? The more detail on this subject the better, because I'm fairly new to all of this. Thanks for your help.

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01-12-2002 08:32:38

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 Re: Phosphoric acid vs. Muriatic acid? in reply to Tom, 01-12-2002 07:43:28  
~Tom, muriatic acid eats both rust & good metal & makes the treated metal rust even more. In many cases will even pit the metal more than it already is & can destroy good parts.
~phosphoric acid attacks only the rust & won't bother most paint or good metal, it actually converts rust to a phosphate & promotes paint adhesion, it goes into the metal pores to kill the rust that is missed by sand blasting & wire brushing.
~The main drawback of phosphoric acid is it won't remove thick rust, grease or paint, so should be used as a follow up to other rust removal procedures like sand blasting, wire brushing etc.
~As for cost, I buy mine by the case & haven't bough it in a while so can't give you a firm price but I believe around $25.00 a gallon. A gallon will go a long ways if just used for a final rust killer & treatment before painting. If you do decide to try it, get the spray bottle they also sell as it allows you to spray it on evenly & actually uses less that way.
~Anybody else that is using a phosphoric acid product from a different company, I would like to hear their opinion on that product. I have had good results with the product I'm using now & am reluctant to change just to try a different product. I'm sure there are others out there using a different supplier of phosphoric acid with different additives in it that have been getting good results also & if I would like to know what brand is being used & how long they have been using it.
~Phosphoric acid is the main ingredient used in "Metal Prep" & other etching primers that is used by many painters to promote paint adhesion on older metal & bare exposed metal that has a tarnish to it.

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buckeye al

01-14-2002 15:25:17

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 Re: Re: Phosphoric acid vs. Muriatic acid? in reply to Clooney, 01-12-2002 08:32:38  
YEARS (over 15) I bought a gallon of Phosphoric acid from a chemical supply house. It's label is now gone & I don't recall what strength it is. It is about like maple syrup. I dilute it about 4 to one & spray it as you suggest. Have used it full strength on severe rust. Give it a few days keeping it in place with paper towels & it will work down into deep pits like on the underside of a platform.

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01-12-2002 09:19:29

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 Re: Re: Phosphoric acid vs. Muriatic acid? in reply to Clooney, 01-12-2002 08:32:38  

I first found phosphoric acid etchant at NAPA, back around 1980. I had to treat some metal that had been in a fire, and this even said for burned metal on the label.

I used to use Metal Prep, but when I ran out of it a few months back, I saw a product at Discount Auto Parts called "Right Stuff De-ruster Formula #3000". It seems to me to me be a bit thicker liquid, but still sprayable/brushable. About the consistency of dish soap. Maybe it hasn't been thinned with water.

I use it after sandblasting to provide the last removal of rust in the pores, and it gives a nice little phosphated look, and the parts seem to stay rust free for months on the shelf. Here in Florida I fight humidity 12 months a year. I'm experimenting with electrolysis cleaning right now, and after the bath, I spray it on, wait a bit, hose it off, and the dry the part completely.

This stuff runs about 8-9 bucks a gallon, and it goes a long way. Sometimes I even put in my old paint gun and just spray it on (USE A MASK !).

The side benefit that you know is that it also removes the last vestiges of silica left imbedded in the metal. Left behind those little silica nodules draw moisture and cause paint and primer to "pop".

They have a website = http://www.cli-rightstuff.com

I'll post the link below.

I like it as well as anything else I've tries, and since it is available locally, I don't have to buy in bulk.

Great for swishing old gas tanks, too. Locks up the old red rust and converts it to the stable form of iron oxide (black oxide). Here's a note I found elsewhere:

Rust is really Fe2O3, a reddish form of iron oxide. Iron has another oxide, Fe3O4, which is sometimes called black oxide, black rust, or hammerscale. Black oxide is a good protection for steel. Like aluminum oxide, black oxide molecules are the same size as iron molecules, so black oxide does not grow or flake. Black oxide is true gun bluing and the oxide found on some drill bits. Black oxide is also seen on iron and steel that has been hot-worked.

I strongly advise folks to stay away from Muriatic acid, as all it will do is erode the base metal, and the iron chloride residual it leaves behind can remain corrosive after the paint is applied just waiting on a scratch to find mioisture !

Hope this info helps !

Frank 70D/855


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01-12-2002 09:18:10

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 Re: Re: Phosphoric acid vs. Muriatic acid? in reply to Clooney, 01-12-2002 08:32:38  
Thank you, I'm glad I asked. I let a starter bracket with end cap from my H soak in muriatic acid for a few days once and the sheet metal (end cap, etc.) came out REALLY thin. I just thought that there was more rust there than I thought. Now I know not to use it again. I only used it in the first place because someone on Rummy's mentioned it many months ago. Thanks for the warning, wish I had known earlier, my starter bracket would probably not be so thin. Also, thanks for the plethora of knowledge you so graciously share with the rest of us in this hobby. I always look for your posts and I really appreciate the help.

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