Phosphoric Acid; how long to dry?

[mhorowit]

I've begun removing rust from my fuselage with a wire brush on an angle grinder; finished one bay, wiped it down with alcohol, let it dry, then applied Phosphoric acid. After 2 hours it's still tacky. In your experience how long should it take for the acid film to dry? - Mike

[Pearson]

Re: Phosphoric Acid; how long to dry?

Mike,

I don't have a bottle of the stuff I use handy. So I can't refer to the instructions about how long you leave it on. But I do recall that it is only for a few minutes. I believe I usually leave it on for about 15 minutes. And I'm sure you are not supposed to let it dry.

My suggestion is to go wash whatever you have coated with the acid, and wipe it down with an old rag. Be aware that all the water you rinse it off with will be considered hazmat by the EPA. So if you can do it without letting it drain into a stream it would be best.

Were there instructions on the product you are using. All of the chemicals you will be using during the restoration process have serious consequences if you don't follow the instructions and warnings. If you have a container where the instructions are not readable, go to where they sell it and find a container with instructions. Or just call the store. They will tell you how long to leave it on.

Also be very aware of the health aspect of using these chemicals. I have a big box of nitrile gloves that I use when using a lot of this stuff. Often the chemicals will eat through the gloves and I have to wash up and put on another pair. Dishwashing gloves don't last much longer and are much more expensive, so I just use the thin nitrile ones. Always wear a respirator, not a dust mask, when using any chemical with a high VOC (volatile organic compound) such as paint, paint stripper, glue, etc.

Keep in mind, phosphoric acid doesn't really remove the rust. It converts it so that it stops eating the metal. From your earlier posts I saw that you are going to have it sandblasted. Sounds like you might be just trying to remove the flaky rust so you can see what is underneath. I suggest you use some 80 sandpaper, or a coarse scotchbrite pad. Replace the obviously bad tubes and go get it blasted. Then you can check if anymore tubes need to be replaced. Even with a thin coat of primer, you will see where any pitting is.

[Robert Lees]

Re: Phosphoric Acid; how long to dry?

The Orthophosphoric acid I used to use (before I got a sandblaster!) required dilution with water (exothermic) to get it to do anything. It was reasonably unpleasant stuff.

It required washing off with water BEFORE the film dried. The water started the rusting process again.

I'd suggest it serves no useful purpose in aircraft restoration.

[Ray36048]

Dupli-Color Primer

I have been using Dupli-Color self etching primer. It comes in a spray can. Sandbalast steel parts. Then spray them with the primer. The primer will prevent the parts from rusting. After everything is done, set up and spray epoxy primer over everything. Works on both steel and alu.

[mhorowit]

Re: Phosphoric Acid; how long to dry?

The Orthophosphoric acid I used to use (before I got a sandblaster!) required dilution with water (exothermic) to get it to do anything. It was reasonably unpleasant stuff.

It required washing off with water BEFORE the film dried. The water started the rusting process again.

I'd suggest it serves no useful purpose in aircraft restoration.

Hmm - several places have mentioned using phosphoric acid to remove any rust in pits. Instructions are pretty clear and do not say anything about removing the film - Mike

[Robert Lees]

Re: Phosphoric Acid; how long to dry?

It doesn't remove it, Mike...chemically, it converts FeO2 (iron oxide) into Fe (there are better chemists here than me who would describe it better).

[mhorowit]

Re: Phosphoric Acid; how long to dry?

Let's compromise. How about in makes any existing rust inert?

I don't seem to have done any harm. I was under the impression I could prime over it.

An hour ago I dipped some 4130 in the acid and will see if it drys overnight; I'll try and see if denatured alcohol is a solvent for it, as the idea of re-introducing moisture to my work isn't attractive - Mike

[flyguy]

Re: Phosphoric Acid; how long to dry?

My comments from using phosphoric acid etch on automotive projects: Always the automotive applications say to wash it off with clean water and prime immediately as soon as dry. I have left it on for a while on some small parts just for the heck of it and it always turns into a kinda gray goo on the surface, similar to your results. I have never seen any instructions that said to leave it on.

On a further note there is a product on the market that is the ONLY one that I have ever seen that will stabilize and convert the tiny rusted pits left over from sanding or blasting. It came out from Permagasket (I think) then was bought out by Napa Autoparts and then came out from several manufactures. It converts the remaining bits of rust to a hard black coating with a clearcoat over it. You can leave it set for a long time as long as it doesn't get condensate or water directly on it. Requires 24 hours for the conversion and complete drying. Then you just prime over it. They suggest light sanding.

Comes in a spray can or can be brushed on as a kinda white coating that will turn black overnight. You can find it everywhere. Just a little of the brushed kind goes a long way. Ask for "rust converter."

As I say I have never seen anything else that even came close for solving this problem.

The brush on type I have in the garage says it is from W.M.Barr and Co., Inc and is labeled Rust Converter. See kleanstrip.com on the net.
Try it on some rusted pipe or or barstock, you will be amazed.
Darryl

[mhorowit]

Re: Phosphoric Acid; how long to dry?

All - I've almost talked myself off the ledge.

When I get home tonite, I'll see if I can remove the film with alcohol. If not, then I'll try water followed by alcohol to displace any remaining water.

If/when I add water to the film, will I have "rehydated" the phosporic acid? - Mike

[mhorowit]

Re: Phosphoric Acid; how long to dry?

Mike,



My suggestion is to go wash whatever you have coated with the acid, and wipe it down with an old rag. Be aware that all the water you rinse it off with will be considered hazmat by the EPA. So if you can do it without letting it drain into a stream it would be best. .

I"ll try alcohol first, then water.

Were there instructions on the product you are using. .

Yes, and they don't say anything about wiping the goo off!!! Just says wait 1 1/2 - 2hours and prime, but it's too gooie to be touched with a brush.

From your earlier posts I saw that you are going to have it sandblasted. Sounds like you might be just trying to remove the flaky rust so you can see what is underneath. I suggest you use some 80 sandpaper, or a coarse scotchbrite pad. Replace the obviously bad tubes and go get it blasted. Then you can check if anymore tubes need to be replaced. Even with a thin coat of primer, you will see where any pitting is.

Thanks; I'm using a wire brush on an angle grinder to remove paint/primer down to bare metal; augmented by stripper where the wire brush won't reach.
Please see another thread I'm about to start concerning a less dramatic approach.

[Hank Jarrett]

Re: Phosphoric Acid; how long to dry?

I would assume you had reactivated the acid when you added water (or alcohol, pure alcohol is not usually available and most "drug store" alcohol has significant amounts of water in it).
Treat what you clean from the metal as hazardous and it WILL eat holes in your clothing (and your skin if you can stand the burning)!
Rinse the tubes as well as you can and blow dry with a compressor (WEAR EYE PROTECTION!) The air will blast the water all over the place and the drops may be corrosive. DON'T get it on other things you want to keep. If the EPA finds you doing this they will have a FIT and the fines are significant.
I don't like using Phosphoric Acid unless I have the facilities to use it right (tanks for dipping, multiple tanks of clean de-ionized water for rinsing, a drying facility and immediate prime and paint facilities) that we had when I did this work for the Navy. It is unbeatable when used with the right facilities, but I don't think it is worth the effort for a garage or hangar project. I dry blasted, wiped down with thinner (take explosion and respirator precautions), tack ragged, primed and painted my 45. The 45 fuselage has been in the barn for a decade and shows no corrosion.
Hank

Stay safe, don't get burned or blinded (by the acid or the EPA) and get the stuff off your plane. You may also want to check with a stone mason for a glove source. They use acid to wash stone and their gloves are made to stand up to the acid. Also keep some pails of clean cold water to pour over any places you get splashed with acid. A full flush with clean water may be the only thing between you and permanent eye damage.

[N96337]

Re: Phosphoric Acid; how long to dry?

Exactly what product are you using Mike???? It sounds to me like there is something missed on the directions, or they have incomplete directions. There should be a website or phone number for the mfg...call them and ask them specifically about this procedure. It's no time to me screwing around and guessing, or saying "this works for product "x", so it should work for this".

As far as a brush on conversion, I've been usuing Stewart's Aircraft Finishing Metal Conversion, and it's awesome stuff!
John

[Ragwing nut]

Re: Phosphoric Acid; how long to dry?

Phosphoric acid should be diluted 10:1 with water. Get a rubber glove that will withstand the acid and wash the tubing down with water. If you do not immediately prime it as soon as it is dry , it will start to oxidize and surface flash rust. There is several products on the market you can buy ready to apply that kills the rust and encapsulates it, and turns any live rust black.

[mhorowit]

Re: Phosphoric Acid; how long to dry?

Exactly what product are you using Mike???? It sounds to me like there is something missed on the directions, or they have incomplete directions. There should be a website or phone number for the mfg...call them and ask them specifically about this procedure. It's no time to me screwing around and guessing, or saying "this works for product "x", so it should work for this".

As far as a brush on conversion, I've been usuing Stewart's Aircraft Finishing Metal Conversion, and it's awesome stuff!
John

Problem is solved: it's coming off, hopefully with alcohol, if not, water; then I'll continue with the Stewart primer - Mike

[mhorowit]

Folo: Phosphoric Acid; how long to dry?

Took yesterday's sample and used alcohol to wipe the goo (which had hardened somewhat) off, leaving a matt finish on the sample.

Water may have worked just as well. - Mike