These short videos show both Iron contamination and tar.
Iron contamination is made up of a lot of things - On your wheels it's brake dust and metal components sticking to your wheels and on the body work, it's 'industrial fallout' - again picked up in many ways i.e dust from train tracks, particulates on the motorway etc etc. - You can be guaranteed that if you have never 'decontaminated' your car, you will have iron deposits on your paint. - this has also been proven when used on a brand new car which picked up contaminants from travelling & storage whilst awaiting delivery to the showroom / customer.
In this video you can see how these fallout removers work with embedded brake dust on alloys wheels - It's normally the yellowy/browny looking deposits that sit on the insides of an alloy that won't shift under normal wheel cleaning.
The video below shows the product 'Bleeding' - what this is the chemical interaction on the iron deposits - it's effectively removing the iron. - A very clear indication indeed that the product is working. - A disclaimer on the Iron fallout products is that due to it's chemical makeup - they absolute stink !! - Think of dead roses in an egg farm, lol. But they are outstanding effective
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Tar Removal.
Over the years there have been many products to use for Tar removal - including WD-40, petrol, white spririt etc. - Again over the last few years there have been a few products that have excelled in this treatment.
One of them that I'd certainly recommend is Autosmart Tardis - you may have seen the autosmart vans driving around your town - They cover nearly the whole of the UK and you can stop them and grab a pot of this - an essential item in your armoury.
As well as tar removal, Tardis is an excellent product to remove chewing gum and sticker residue, i.e stickers glue on glass etc - It simply is a spray, leave and wipe product. I've even used this on one of the car interior mats when the youngest decided to drop bubble gum on it.

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With both of these products, it's essential that a full rinse down is done after the chemicals have been applied and also bear in mind that as they work very effectively, it may be worth taking care especially in any recent 'smart repaired' or recently painted areas on your vehicle.
Use in moderation and if you are unsure, test on an inconspicuous area first.
any Q's you may have - let me know - Glad to help as always

Jay