Dont try and use straight phosphoric acid!! It needs inhibitors in it. I think it is also blended with acetic acid, not certain though.
I remember reading in a very old 'farm remedies' book formulations for rust conversions, there was also had a recipe for re doing gun metal finish on fire arms.

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Grumpy Old Diesel Owners Club group

I no longer question authority, I annoy authority. More effect, less effort....

1967 230-6 auto parts car. rust bucket.
1980 300D now parts car 800k miles
1984 300D 500k miles
1987 250td 160k miles English import
2001 jeep turbo diesel 130k miles
1998 jeep tdi ~ followed me home. Needs a turbo.
1968 Ford F750 truck. 6-354 diesel conversion.
Other toys ~J.D.,Cat & GM ~ mainly earth moving

remove what? the only things left in my interior are the hvac main unit, and the steering column. The drivers seat is also still in.............. not quite sure why though...
I am going to get my room mate to show me how to use the grinder here before too much longer, then its time to get some work done.
Andrew

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83 300D "shoulda been a parts car"

If an acetylene torch can be safely applied to the area that is rusted, barring any flammable material in the vicinity of the rusted spot, then the rusted area can be brought to the red heat of iron - not melted and properly supported to remove stress.

Once the part is red hot then the torch can have it's oxygen turned down to carburizing flame. The extra carbon in the flame will cause the oxygen present in the rust to be liberated and bonded with the carbon. (Caution as carbon-monoxide as well as carbon-dioxide will likely be present in the resultant gas.) The rust will be converted back into it's original iron. Do not overdo the carburization process as the iron will then be converted into a high-carbon steel and be brittle. Both sides of the metal part must be treated in this fashion.

While the metal is hot and the rust converted, the metal can be worked into shape and galvanized. Bronzing of pitting is also possible though the electrolytic action between the metals is problematic. Zinc is much better as a rust preventor.

I've not used this method on my car but the science is sound and I have not found any rust yet.

Above all, no flammables of any type should be anywhere near the area!~

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Codifex
1981 240D ChinaBlue (Got her running with a donor engine.)
1983 300DTurbo w/sunroof.
1984 300TD manual sunroof. (Electrical Gremlins)

^ that sounds like it could definitely be a very fun way to get rid of rust. Sadly, that much gas would be a little expensive. hopefully the por 15 will be able to get the job done.
Andrew

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83 300D "shoulda been a parts car"

The big improvement in rust resistance is in my opinion, galvanizing. It took a long time to figure out how to get a coating to stick to it for any length of time. I lived in the rust belt for 35 years and the sight of rust on a car physically sickens me.

POR-15 is expensive if you are going to do something big; I've been happy with SEM Rust-Shield, which isn't the same chemically as POR-15 but I've been pleased with the results - I used it preventatively as well as during repairs. So far, so good.

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86 560SL
With homebrew first gear start!
85 380SL
Daily Driver Project

http://juliepalooza.8m.com/sl/mercedes.htm

Never apply phosphoric, acidic or any other strong acid to metal. It will greatly accelerate the rust. The first treatment on metal is to phosphate it. The most basic of this is iron phosphate. One of this material's trade name is Bonderite 1000. There are many variations of the phosphate (not phosphoric--totally different beast).

Using the torch to reduce iron oxide to iron. Don't really think that is what is working other than the torch removing the iron oxide (iron oxide is the chemical name for rust on steel). Iron oxide melts around 1400-1500 C and oxy/acetylene flame is around 3000 depending on gas mix. I think you are melting (or burning) off the rust with the torch. Mechanically removing the rust would work as well. Now I am not saying that the reduction does not occur. This technique is used by welders for many applications. It is just that automotive corrosion is quite severe and the gauge of the steel in cars is quite thin. You really cannot obtain significant corrosion protection just by reducing the iron oxide.

There are other reasons using the torch has it issues. High heat can enbrittle metals and change their mechanical properties. Experts with torches know what they are doing but still must consider this. This would be an issue if you are working on a load bearing member. Plus CO (carbon monoxide) is quite dangerous. Reducing flames can produce a lot of CO.

Now what I think is the more important than the heat for rust reoccurrence is the zinc. There is another metal treatment that is zinc phosphate. I have found it to be more effective than iron phosphate for pretreatment of metal. Zinc is also used in many galvanization processes.

Phosphate treatments on their own will not prevent rust. They must be coated with anticorrosive coatings. the corrosion resistance actually comes from the coating, but phosphating allows for significantly better adhesion of the coating. (it is more complex than this, though trying to explain galvanic processes is very difficult, confusing, and time consuming.)

POR-15 is the popular one on this website. I have never used it myself, but many swear by it.

Rust prevention has a lot to do with adhesion. The reason for this is actually physical. When steel converts to iron oxide (or rusts), it expands tenfold. This causes the material to delaminate the coating. The oxide also then falls off the steel panel and exposes fresh metal to the corrosion process. This is why aluminum does not corrode as severely. The aluminum oxide does not expand. It actually stays on the aluminum surface and prevents further oxidation.

The last comment on adhesion is probably the most important factor to prevent rust.

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2010 Toyota matrix

'93 500 SEL
A bad addiction. Takes all of my cash.

'12 Volvo S80 T6
Needed something that wasn't as hard to deal with as my bad addiction

'18 Mazda Miata
No more boring cars for everyday transport!

so i played with the grinder for about 4 hours today, and about 90 % of my firewall rust is removed. I need to figure how to get a cutter up between the Hvac and the firewall behind the sway bar mount. its amazing how long it took to get the small amount of metal i removed out.
Andrew

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83 300D "shoulda been a parts car"

the rust on the firewall is out, all that is coming at least. I am going to start putting in new panels pretty soon, but i am wanting to ask what kind of measures i can take to prevent rust on the metal that is going to be against the other sheet metal. such as when i get around to doing the outer rocker, how do i coat the inner part of the outer rocker. I would think paint, but standard paint would just get burnt off by welding.
How well does POR-15 hold up to welding heat? ex. how close to por-15 can i tack weld without worrying about destroying the coating?
Andrew

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83 300D "shoulda been a parts car"

You can use a tool like this to inject anti-corrosion solutions into closed areas like the rocker panels.

http://www.eastwood.com/rustproofing-and-undercoating-system.html

For lap joints, you want to use a weld-through primer.

any particular weld through you would recommend?
Andrew

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83 300D "shoulda been a parts car"

I think I picked up a few cans at Napa the last time I needed some. I'm not sure what brand it was.

Alright, i have the firewall passenger side patches made, my roomate is welding for me so it is coming, i started looking at the car today and it seems like the passenger side B- pillar, rocker and rear floor pan have been replaced from another car. the paint is different color and it is just magically rusting out at the places where the new panels meet the old lol.
But i have seen that there is some rust just above the rear suspension front mounting point. I sm thinking i might just go ahead and pull the rear suspension out of the car when i get back there to truly fix it. i also have rust in the trunk floor pan, so i need to get back there anyway.
How hard is it to drop out the rear suspension? just looking at it, it looks like the drive shaft, mounts at the front, the differential mount, brake lines (hydraulic and emergency), and the top of the shocks. Is there anything else i need to remove?
I know this sounds a little crazy, but at this point i might as well fix this thing properly. I can also give the suspension a good look to see what parts i need to replace.
Andrew

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83 300D "shoulda been a parts car"

Start taking some pictures, I want to see this. And on the topic of the weld-thru primer, is it really necessary? I hate to redo the spot welds, because they are the most susceptible to rust and always the first to go, but I'd like to restore to factory design and doing the spot welds would also be easier. Do I need to use the weld-thru? It's pretty expensive.

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1983 240D 3.0T 4-speed manual, now sold

1989 Subaru GL Wagon 5-speed Touring Edition

I am only using the weld through primer on the parts that i cannot get to the back of, i am basically replacing the passenger side floor pan and rocker, so most of that will be finished in a much more conventional method. I have a spot of rust under the front sway bar mount, so i am using the weld through.
As far as pictures go, here is my flickr account, most of these are rather old at this point. if i get the chance to work on her this weekend ill put some new ones up. I just found out i have to get a poster report done by sunday night, and im working all weekend so who knows if i will get some time lol.
Andrew

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83 300D "shoulda been a parts car"

got some new pics up in the mercedes rust
http://www.flickr.com/photos/38963910@N06/sets/72157623120725463/

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83 300D "shoulda been a parts car"