[abrogard (OP)]

 I read the thread about Vinegar removing rust.

 I need to remove some rust from a car body panel where I've welded in some new stuff but left it out in the rain and the welds themselves and part of the new steel all got rusty (even though I sprayed it with undercoat).

 Vinegar sounds great. But I think you must soak it in vinegar for some time, right? I can't just rub with vinegar and expect it to chemically alter the rust, can I?

 Well, I can't soak the car in vinegar.

 the thread mentioned ketchup as a good alternative... I think that's out, too.

 So maybe I need to make a paste of the vinegar, somehow, and smear that on.

 So how would I make a paste for this purpose? i.e. a paste that doesn't neutralise the vinegar?

 And that old thread mentioned that the effect of these weak acids is also to coat the metal with a coating that makes further rusting a little harder to do.

 So I wonder about that and washing? I suppose to apply modern auto  paints or undercoats onto such a surface after the vinegar treatment one would have to wash all the residue vinegar off?  Definitely have to wash the treated (black sludge?) rust off, rather than a good wipe, I guess?

It would be so much better if the treated rust, wiped down to the thinnest layer, could be painted over, wouldn't it, rather than introducing water into the mix.

But all of that gets into the chemistry of today's paints, I guess...  too esoteric for this forum or not?

regards,

ab   

[Don_1]

Hi & welcome to the forum.

I'm no chemist, but I think ZnO (zinc oxide) would probably be the best thing to use for this purpose. ZnO is partially soluble in water and is soluble in most acids. It is widely used as a base for sun creams and the like. Being a powder, you can dissolve as much or little as you require to get you paste consistency right for your purpose.

If you can get hold of some dilute acetic acid into the bargain, so much the better. I'm pretty sure acetic acid is used by the motor industry to etch metals prior to painting. If you can't get acetic acid, then it might be best to use clear vinegar rather than the malt usually used sold by the food industry.

With a bit of luck, someone more knowledgeable in the chemistry field will be along to give you a better idea, in a short while. You need our friendly Bored Chemist.

[CZARCAR]

phosphoric acid or "RUST TREATMENT' primer

[Bored chemist]

"ZnO is partially soluble in water and is soluble in most acids. "
And, in dissolving in them, it neutralises them.
Cornflour would be a better bet, but messy to wash off afterwards.

[JP]

I did a flour/vinegar solution to clean oxidation off a decorative metal plate at one point.  It worked pretty well in combination with some steel wool and a toothbrush.

[Geezer]

I know it's not an easy task, but your best bet is to remove all the rust mechanically (sandpaper, sandblasting, grinder, etc.) Based on bitter experience, if you don't do that, the rust is likely to "bleed through" the paint sooner or later. If you don't need it to last too long, go with Don's suggestion and use a zinc primer.

There are proprietary products available that might solve the problem chemically, but I've never had much success with them.

[abrogard (OP)]

 I agree with you - the rust 'likely to bleed through' - but that's the whole point of this approach. To remove the rust with a 'rust converter' such as vinegar or phosphoric acid, I think it is, that they use in the commercial mixes.

 Because I'm thinking that these fluids or pastes will squeeze into all the cracks of the welds and joints and remove every last speck of rust - stuff I might miss with a grinder. And then it will be all gone. And can't come back. Can't 'bleed through'.

 Are you technically savvy with the physics/chemistry here and can tell me that it just won't work?

 If that's so I will abandon the approach now and forever, I mean for future jobs too.

 

Edit p.s.   Sorry, didn't see the earlier posts.  I think maybe the toothbrush and steel wool thing is a physical removal thing, mainly. I see phosphoric already got a mention. In fact I've used it. Did a whole 6 x 4 rusty trailer with it. The rust is coming through again. That was a VERY rusty trailer, true, but it still maybe indicates that rust cannot be 'Stopped' in this way.

That's what I'm hoping to find out about in this thread.

Washing off cornflour raises the question of water on bare metal. Especially into little cracks and such that you get in (admittedly bad) welds such as mine. I'd much rather a preparation that didn't need washing off. If one must wash off then how likely is rust to have already gotten a hold even though perhaps unseeable at that stage, I wonder?

[Geezer]

Are you technically savvy with the physics/chemistry here and can tell me that it just won't work?

Nope - I'm just an engineer.

My input was based on personal experience.

[abrogard (OP)]

I think I'm going to have to get some phosphoric acid, or a commercial product that uses it.
I've done a lot of googling. Vinegar would work well if it could be soaked. It will then get down to the individual molecules of iron oxide, which is the kind of mechanism I was thinking about - get every last bit that way, don't you?
 But it can't be soaked.
 Phosphoric will be quicker, I guess, is the point.  Seems the right way to go depends very much on your particular rust problem, including the nature of the metal you're dealing with. No one size suits all.

 Thanks for your interest everyone.

 

 I did read one thing that interested me a lot: molasses and water. A molasses and water mix was said to be a great rust remover. The writer claimed the secret was that it produced its own phosphoric acid.  I'd like that. Appeals to my sense of humour, fixing my car with molasses....

[lightarrow]

Vinegar sounds great. But I think you must soak it in vinegar for some time, right? I can't just rub with vinegar and expect it to chemically alter the rust, can I?

No, infact, you should let the vinegar act for many hours.

Well, I can't soak the car in vinegar.

It depends on how much vinegar you have  []

So maybe I need to make a paste of the vinegar, somehow, and smear that on.
 So how would I make a paste for this purpose? i.e. a paste that doesn't neutralize the vinegar?

I used a concentrated solution of sugar in water (you have to heat to dissolve it completely) to form a very viscous liquid. I have used it with HF, instead of vinegar, but the principle is the same. I would add lemon juice, to the vinegar, anyway .