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unstable 07-20-2003 07:34 PM

Phosphoric Acid + Prevention of flash/surface rust
I just wanted to let everyone know about my experience in using phosphoric acid (metal etch/floor etch/metal conditioner).

A gentleman on the Hobart Welder forum recommended using it to neutralize rust.

I've got most of my sheetmetal in my basement (being that I do not have a garage). It takes time to manually strip through all of the years of paint, primer and filler. In the meantime, surface rust has appeared on many of my panels. It really doesn't take long for it to rear its' ugly head, 3 days at the most around here.

So I picked up some stuff called "Floor Etch" which is just phosphoric acid, and I've layered it on the bare metal...and I just have to say, the stuff is phenomenol.

Wearing rubber gloves and protective eyeware, soak a rag in the acid. On light rust, it will actually wipe it away in a few least it appears to.

On non-rusted bare metal, it puts a sort of grey-frosting on it. TO my understanding, you should actually hit bare metal with this stuff, it helps the primer adhere.

On somewhat thicker rust, I just pour the acid right onto the metal. If the panel doesn't have any holes in it, you can tilt it around and get full coverage. It will turn the rust into a hard black substance...not sure what it's called chemically.

Another good method is soaking a rag, smearing the stuff around, and leaving the drenched rag on the metal. The rusty areas will turn black.

I've also used a wire brush when I'm applying the acid. Kind of brush the acid around with the brush and at the same time you're brushing away little chunks of cancer.

At the end of the day, it APPEARS that this stuff at least SLOWS the rusting process substantially. I cannot say whether it completely stops the rust...I'll let you know in a few years.

I recommend at least trying this stuff where you would not use por-15. It's cheap..and I just wish I would have known about it sooner.

Hope this helps someone out.

crazy larry 07-20-2003 08:25 PM

por15's METAL READY is a zinc-phosphate etcher, and i've left parts unattended for weeks without ANY rust appearing.......

unstable 07-21-2003 07:12 AM

metal ready is a tad more expensive. Price difference is neglible. I'm just putting the information out there for whoever is interested.

asennad 07-21-2003 08:36 PM

The problem with PA is that it only gets the surface rust. If the rust has any thickness at all it will only change the top layer of rust and leave the sub-surface untouched. I would hit any rust with a 60 grit sanding pad to ensure that there is no rust left to rear it's ugly head down the road.

jerry hobgood 11-14-2015 10:23 AM

'31 Roadster Gas Tank in Place Rust Removal
So how did that work for you? I'm in need to flush out my 31 Roadster gas tank in place with something and am interested in the phosphoric acid in lieu of muriatic acid. Then neutralize somehow.
Thanks for your thoughts as I want to act as soon as possible.

Pats55 11-14-2015 05:30 PM

Metal prep is your first line of defense against corrosion. It has been for over 30 years that I know. Use a phosphoric acid solution that contains zinc phosphate.There are a lot of quality metal prep's that will dissolve heavy surface rust Do not use muriatic acid in a gas tank. Muriatic acid keeps on working and if you put a sealer over it there's a good chance it'll delaminate. Floor etch is usually muriatic acid. I may be mistaken.Read the contents Never rinse phosphoric acid off with water and let it turn orange and then prime and paint over it. If you have to rinse it used denatured alcohol. I've used acids since 1974 without any problems.

jerry hobgood 11-14-2015 06:15 PM

Thanks. But my tank is still installed in the Model A and I have limited ability to shake the car, etc. I do have a chain that I was going to move back on forth on the tank surfaces. If I rinse with denatured alcohol can I then refill a clean, rust free gas tank, with new gas? Appreciate your input as I hope to get all this done next Sat. before any freezing weather in MS.
1. Time to have phosphoric acid in tank topped off?
2. Rinse once with denatured alcohol in tank topper off?
3. Fill gas tank with ethanol free gas mixed with Marvin's Mystery Oil.
4. Process complete!

John long 11-14-2015 06:15 PM

For those who are not experienced with acid, be advised. Any residual acid left on a panel will keep activated paint from curing. You will end up with blisters, delamination, or other issues if you try to paint over the acid. Yes, it cuts the rust but it MUST BE removed before you paint by re-wetting with acid and rinsing well.


Pats55 11-14-2015 06:44 PM

Hello Jerry, sounds like a good plan. Usually on a model a tank after it's been treated we spray some type of sealer to prevent the rust from returning. A engine cleaning gun from Harbor freight usually works well for this Your idea of fuel with Marvel mystery oil and probably work out also. As for acid is spent 25 years behind a table at all the major car shows on East Coast very few problems with acid. This is the first place I hear about all these problems with acid. What is the point of D rusting something rinsing it with water until it rusts and and painting it, this is all beyond me. As long as you don't have a lot of zinc phosphate on the metal there should not be a problem with adhesion.

jerry hobgood 11-14-2015 06:59 PM

I'd prefer NOT to spray a sealant into the mounted A Model gas tank and just keep the tank topped off with gas. I number of people have had poor results from gas tank sealers!
Your input again welcomed.

Pats55 11-14-2015 07:02 PM

I can relate to that. One thing about a gas tank you don't want to screw it up because it will turn out to be a nightmare.If the Marvel mystery oil trick works that would be great. If it doesn't work you know we drain it clean and then seal it as a last resort. Good luck

Lizer 11-14-2015 08:50 PM

Nice bump to a 12 year old thread.

deadbodyman 11-15-2015 07:39 AM

I wonder if it worked , he never got back to us:confused:

jerry hobgood 11-15-2015 08:09 AM

Yes, that WOULD be good to find out and post if a friend of his could make contact. I'd never not to keen on using acid in my fixed Model A gas tank, as my
only radical option is to remove tank, split, etc., major ordeal in my book.
BTW, I love my Roadster!!! :)

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