Iron Remover

The active ingredient in most iron and fallout removers is ammonium-thioglycolate.

https://canadiangearhead.com/iron-remover/

https://www.amazon.com/Adams-Iron-Remover-Dissolves-Particles/dp/B07C2YM4LF

https://pubchem.ncbi.nlm.nih.gov/compound/Ammonium-thioglycolate#section=Vapor-Pressure

https://en.wikipedia.org/wiki/Ammonium_thioglycolate

https://www.sigmaaldrich.com/US/en/product/aldrich/10010

https://www.researchgate.net/post/how_to_make_ammonium_thioglycolate_60_solution_from_thioglycolic_acid_80_and_ammonia_30

I....

Might be able to help.

I spent close to 2 decades playing around with chemicals (still do, in fact). Manufacturing, formulating and all the other fun stuff that goes with it. Primarily what is called HI&I products- household, institutional and industrial- with a major focus on janitorial and car care products.

One of those was a fallout remover. Another was a clay lubricant. And also ammonium biflouride based rim cleaners among other things.

First thing about fallout- it's not just iron, but iron is the primary culprit because there is a lot of it and of course it turns that lovely orange colour as it rusts, leaving tiny orange stains that gradually become more visible as it bites further into your beautiful paint job. This is from brakes and also rail dust- you'd be surprised how much of a problem this can be on brand new cars!- as well as fly ash and a handful of other things of lesser note.

What happens is that the reduced iron particles land on the surface. To be clear, we are talking microscopic to barely visible in most cases, larger particles will wash of more easily. With time and moisture they oxidize through the ferrous to ferric stages, moving slightly and growing in size as they oxidize and bind moisture to form the typical ferric hydrate compounds of rust. As this happens they also work their way farther down into the paint surface, doing permanent damage.

Now from a chemical point of view! First off let me address your last comment, namely using hydroflouric acid. I have a very good safety tip for using it, as well as ammonium biflouride based products!

Don't.

Just don't.

I don't care what anyone else on here says or thinks when it comes to this. I personally manufactured many thousand gallons of commercial products and oversaw far more than that, including ammonium biflouride based rim cleaners as mentioned. Even so, I would have to complete a 2 day course before my suppliers could sell me concentrated HF, if I was interested in buying it. If you are interested in possible amputations of digits or limbs, gangrene and decalcified bones then try it. For me, the reason my avatar has nitroglycerine is because I'm too scared to use HF!!

Back to your main question. As a thioglycolate- the link you provide says it has a high percentage of sodium thioglycolate- it does NOT primarily oxidize the iron. First off, iron cannot be practically oxidized beyond the ferric (Fe3) stage in normal life so the only thing it can do is oxidize remaining ferrous material to ferric. Second, thio just about anything tend to be reductants to most thing. Third, as another poster points out generally oxidation tends to expand not shrink things. In the case of fallout the major reason for this is the expansion of water of hydration.

What happens is that the sodium (a spectator ion, having little or no activity in this process) helps to reduce the nasty smell you get with the raw acid. When you spray it on it reacts with the iron in a displacement reaction, forming iron thioglycolate- that's what gives you the red or purplish colour.

This is somewhat water soluble, so it can be washed off as well as providing visual indicators that marketing people love! The agitation and chemical dissolution of the surface of the iron help to loosen the iron from the surface and allow it to be washed away. This is part of the reason for the other components of these products, such as glycol ethers and surfactants to help lower surface and interfacial tensions, increasing wetting and removing particulate through detergent action.

The products we made for this were acidic- it just works better and faster!- but based on organic acids and chelating agents, not thioglycolates. This provides a product that is relatively safe to the vehicle, acting as chelation ligands (this means the product forms an ionically oriented linkage that can solubilize and suspend the metal, as opposed to an acid/base reaction) to remove the iron.

As far as cost goes, it is a bit of a specialty item and you are paying for this. Plus the fact that it typically has a far higher percentage of active ingredients, and someone has to pay for the marketing campaign, the company owner's boat....

Ultimately a professional product is the best option, but it's still a slow process if you don't want to risk damaging the surface. And I would not recommend using it as a clay lubricant. They have one purpose only- to make the clay glide. The clay is to pick up the surface contaminants, and absorb them into itself as you use and work it. The lubricant makes it so these do not scratch the surface as you do this, and these products will not do anything for fly ash and anything other than just the iron.

Hope this helps!