Acid Types

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PostPosted: Sun Sep 06, 2020 12:17 am
Hi All,

So I am a big fan of phos acid.

However, there are some instances in which phos acid liquid, naval jelly, vinegar, or evaporust will not touch the rust, especially in pits. Regardless of if a steel wheel, or bristle brush is used.

I've been doing some research and see that muriatic acid is considered a nuclear option, and is well received by the old timers.

I have seen discussion of hydrogen embrittlement, or talk that once you use muriatic on the panel it will forever rust (something to do with chloride ions). I've been considering purchasing some, neutralizing it, and following up with PHOS acid to prevent the flash rust and then neutralizing that.
I have also seen favorable reviews on citric acid.

So my question, who uses muriatic acid or citric acid in your auto body work?
Have you found any issues with permanent rusting?

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PostPosted: Sun Sep 06, 2020 8:08 am
Well, I'm in the chemical end of the business and I'll be the first to say I think exposing metals to ANY chemistry can be very tricky and even further, disastrous. The problem generally isn't on the exposing side to the acids... it is the assuming you've got it all dry/neutralized. You should take a good look at metals under a magnifying glass and further under a microscope. The nooks, crannies, etc., can be impressive, then take that observation to the folds, creases, seams, interior volumes of panel, etc. Modern epoxy primers can literally be turned off from drying/curing if there is the presence of any of those acids you mentioned.
There is a good reason you will see a lot more about media blasting. Even if minor residue is left the epoxy primers just encapsulate it, no reaction. At the archival level of restoration with woods and metals they are dry ice blasting now. Stuff hits under pressure, sublimates directly to gas.....poof!, no signs you were even there.
Not saying you can't play with acids, but if it was any project that I cared about I'd go blasting..... Others will chime in......
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PostPosted: Sun Sep 06, 2020 9:16 am
I have used muriatic and wouldn't recommend it for anything automotive.
If you live near Toledo or Cincinnati, American Metal Cleaning will get all the rust off your panels without any harm to the metal. They use Electrolysis

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PostPosted: Sun Sep 06, 2020 7:02 pm
Electrolysis..... yeah, I'm good with that... just hard to find it as a service in smaller metro areas. Customer of mine did a VW bus with that...
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PostPosted: Sun Sep 06, 2020 8:53 pm
i've used phosphoric acid a few times on vintage items that were nearly irreplaceable. i did some doors on an old touring car that had heavy surface rust with some pits. i used the acid on towels that were saturated then placed on the doors then covered with visqueen and sealed around the edges you need to keep it wet. it took about 4 applications and about 5 days BUT the doors came out nice and clean free of rust. i've used muriatic acid to clean various parts i think the rust reoccurrence is because it get the metal virgin clean and any moisture or humidity is going to cause it to rust agian. i used some muriatic acid to clean some small parts that i was going to zinc plate, it worked good but MAY have killed my plating solution. it quit working shortly after i used it. as for the hydrogen embrittlement i wouldn't think it would be a problem with sheet metal if it were a torsion bar or something like that then maybe. i'm with Darrel get yourself a small pressure blaster.
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PostPosted: Sun Sep 06, 2020 10:18 pm
Believe it or not, molasses makes a good de-ruster. It takes a lot of time
though, sometimes up to a month. Get a gallon of feed molasses from
your local farm supply store and drop small parts into it and wait a
couple weeks. The parts will come out completely clean of rust.

Lately I've been using Evap-O-Rust. It does a good job and is easily cleaned
off for painting. I don't think it contains phosphoric acid. You can also buy it
in a jell that clings to parts and won't drip off...
"If you can't move it, paint it." - U.S. Army



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PostPosted: Sun Sep 06, 2020 11:27 pm
Dont use Muriatic acid or Nitric acid it will cause Hydrogen Inbritlement and other issues.
Phosphoric acid is what I use BUT very rarely do I ever use it strait.

I use wire wheels and Glass bead First then use metal conditioners made by Dupont now Axalta or PPG that use Phosphoric Acid in there Base but also contain other chemicals and metals to actually Cold Galvanize the surface of the metal and Prep it for paint when used Properly. Its a two step process.
To use properly you need many hands on the metal you cant let it dry on the metal and it must be rinsed thoroughly, Both steps. The instructions MUST be followed!!!!!

In then end I have used many many Gels and Phosphoric acid products over the years and in the end its just best too Blast with Glass at low pressures and treat with a Cold Galvanizing Metal prep that is approved By a Paint mfg such as Dupont, Axalta or PPG
This product is approved by them to use under their paint system!

Many people strongly disagree with the use of any sort of treatments on metal and in the end I have to say too each his own but I believe in chemistry and science and it works for me!

After treatment with Duponts 5717s
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After Dupont 5718s
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After soap and water wash and allowed to dry in the Death Valley Sun NO Flash Rust!!!!
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Dennis B.
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Line Maintenance A&P Mechanic and MOC Tech specialist.

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PostPosted: Mon Sep 07, 2020 9:32 am
NightTrain wrote:Believe it or not, molasses makes a good de-ruster. It takes a lot of time though, sometimes up to a month. . .


This adds new meaning to the "slower than molasses" saying! :rotfl:
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PostPosted: Mon Sep 07, 2020 12:25 pm
HECK, Dennis there's probably not enough humidity where your at to even think about rust. :rotfl:
i bought a door for a 51 chevy pickup in eastern Oregon ( Burns Oregon ) the door had NO paint on it and No rust. the owner said it had been like that for at least 3 years that he knew of when he bought the wrecking yard.
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PostPosted: Mon Sep 07, 2020 3:30 pm
Humidity certainly makes a difference. Dennis, if you rinsed it with hot water it would dry instantly in your desert home land.

Doright wrote:To use properly you need many hands on the metal you cant let it dry on the metal and it must be rinsed thoroughly

It can't even be partially dry, and water doesn't wake it up, it takes more acid to make it active again. Acid will also soften your scratch pattern on the metal, so acid cleaned bare metal needs a fresh 80 grit DA scratch before paint, that will also get rid of the flash rust.

Here's a test. Use a course media to blast a piece of metal, then stick the metal piece in the phosphoric acid and watch it fizz. It will stop fizzing when all the sharp edges are gone.

I'm not trying to discourage the use of phosphoric acid, users just need to learn all about it before using it. Its like most of the other products we use, learn to use it properly or you'll regret it.
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