I also use Ospho or a similar product. IIRC, it is indeed phosphoric acid and some tannins (tannic acids). My understanding is that it drives the chemical Red-Ox reaction that turns the iron into rust -- Fe2O3 * nH2O -- in a reverse direction. It prys the things away from the water or OH "pieces" and then must lock something else onto those ions.

Maybe some real chemists can comment on this(?) Does it change Fe(III) into a different state?

But how does it create a zinc coating on the surface of clean metal??

It's been a while since I studied unorganic chemistry, but if I recall correctly, what happens when ferrous (III) oxide (i.e. rust) is exposed to an acid, it's being reduced to ferrous (II) oxide, which is black and a much more durable state than Fe(III).

Or, to put it in chemical terms:

Fe2O3 + nH+ --> Fe3O2 + H2O

I am not a chemical engineer but this makes sense to me. What I have been doing is using multiple treatments of Ospho. It usually takes three to five seperate treatments of ospho and grinding wheel to get down to clean metal. I use the ospho to turn the rust black and then grind it all away. Repeat this over and over again until there is only clean shiny metal remaining. I might be being paranoid but this way I am sure there is not rust remaining. Using a wire brush with a puddle of Ospho has also proved to be very effective. Make sure to wear goggles if you do this!

I think I was able to source a floor pan for just the passenger side!!!

Day 28: I was able to make some progress on the driver floor pan and the rear seat bucket on the driver side!

http://forums.pelicanparts.com/uploa...1332817388.jpg
http://forums.pelicanparts.com/uploa...1332817425.jpg

I also sprayed Eastwood Internal Panel Coating into the rockers. I sprayed 1 can into each side, both inside and outside rocker with the special attachment hose to get into the entire thing.


http://forums.pelicanparts.com/uploa...1332817519.jpg

I also was able to grind off a little bit more of the roof

http://forums.pelicanparts.com/uploa...1332817688.jpg

the manf. tells you that it is ok to paint over that black from using the Ospho to convert rust

also found this snippet:
Rust converter - Wikipedia, the free encyclopedia

compare with:

Phosphoric acid - Wikipedia, the free encyclopedia
Rust removal
Phosphoric acid may be used as a "rust converter", by direct application to rusted iron, steel tools, or surfaces. The phosphoric acid converts reddish-brown iron(III) oxide, Fe2O3 (rust) to black ferric phosphate, FePO4.
"Rust converter" is sometimes a greenish liquid suitable for dipping (in the same sort of acid bath as is used for pickling metal), but it is more often formulated as a gel, commonly called naval jelly. It is sometimes sold under other names, such as "rust remover" or "rust killer". As a thick gel, it may be applied to sloping, vertical, or even overhead surfaces.
After treatment, the black ferric-phosphate coating can be scrubbed off, leaving a fresh metal surface. Multiple applications of phosphoric acid may be required to remove all rust. The black phosphate coating can also be left in place, where it will provide moderate further corrosion resistance (such protection is also provided by the superficially similar Parkerizing and blued electrochemical conversion coating processes).

[QUOTE=RWebb;6648996]the manf. tells you that it is ok to paint over that black from using the Ospho to convert rust

I don't believe this at all. I have let the ospho sit and convert rust for the recommended over night period. When I scraped at the black stuff with a screwdriver or went over it with a wire wheel I found rust immediatly under the black layer. That is why I repeat the process 3 to 5 times to make sure I have clean shiny metal.

it is only for a thin layer of surface rust -- maybe that is the problem; also different manf.s may have different "additives" in their products

personally, I've never used the black layer as a primer; I put a Rustoleum primer over it then a Rustoleum paint

why not drop this question in the lap of your graduate students?

[QUOTE=kkinzli;6649474] In my experience also I found out that the "rust conversion" process only works on the very surface of rust. It does not penetrate into thicker rust. Thick rust must be removed with scraping or other mechanical means.

[QUOTE=kkinzli;6649474] Can I make a request? Can you try using Naval Jelly on a section and see if it is better/worse/different? I would suggest leaving it for the same length of time as well. http://forums.pelicanparts.com/support/smileys/chix.gif

check out Wurth Rust Guard...all you need to do is remove the loose rust & crap...I then wipe down w/ acetone & paint it...it says on the can that the product works BEST on metal that has "tooth" = NOT CLEAN SHINNY METAL...
I have never had a rust problem continue in these areas...the finish is rock hard & does not come off...do you guys know of this paint...?? A 1 step process also...
I highly recommend this stuff although it is ~ $70/qt shipped...

We are currently trying to get a graduate program up an running here so no grad students to anwser this one:(. We have only had a civil engineering degree program for five years. I think you are right about the different manufacturers. The reason this topic keeps popping up is that there is so much out there it is hard to grasp exactly which product works the best. Of course each company says their product will save the human race from rust forever, making the task of selecting a product all the easier;)

[QUOTE=davidbir;6650378] I have been toying with the idea of doing just that. Obi Wan mentioned that he uses it on his 356 and I think this would be a worthwhile experiment. I have the perfect place on the front of the passenger pan to try this. I could also do this on the part of the passenger pan that I will be cutting out soon!

How long is your period of evidence? I have the sneaking suspision that rust continues under such products and eventually causes the panel to fail. See Obi Wans comment about POR 15 and his 68 floorpan earlier. Failure will not occur for years or maybe even decades. If you go down to clean shiny metal and epoxy primer ($75/qt) the panel should last longer than the owner. My method takes a lot longer than simply painting over the rust but I feel that it is done right.

Agreed. I know nothing but I think these paint over rust type products are a bit like a bandaid on a gunshot wound.

Well said! If you don't get the bullet out of your flesh you will definitly have problems later on - much like rust. On the surface the bandaid makes things look pretty good even though things underneath are far from ok.

next idea - find a smart undergrad - maybe in Chemistry - and get them to run some analysis on the different brands -- help them write it up...

if it all works out, in a year or two you will have your first grad. student


in the meantime, we can all balance labor input vs. expected longevity

This would be interesting! Maybe do a product comparison study and get a publication out of it ;) I have as many others on the forum decided to use the max labor input = max longevity method.

The Rats younger brother the Black Knight had a huge milestone the other day 250,000 :cool::cool::D:D:)http://forums.pelicanparts.com/support/smileys/wat6.gifhttp://forums.pelicanparts.com/support/smileys/clap.gifhttp://forums.pelicanparts.com/support/smileys/ura.gif

I celebrated by pouring a beer all over the car like I did for 200K

http://forums.pelicanparts.com/uploa...1332961420.jpg

Awesome project. Keep up the good work!

Kudos on keeing the a4 on the road too. My old 97 A4 1.8t went 230k before I put her into a guard rail! I'm positive she would still be running like a champ had I not wrecked her.