Whilst washing is dedicated to the removal of of dirt and non-bonded soils, it is unable to remove more strongly bonded soiling. Bonded contamination is divided into 2 primary groups: Tar and Iron
Tar can be picked up from most road surfaces and by its very nature, it is extremely sticky and hard to remove. It is totally insoluble in water and will not be removed by regular washing. Whilst it can be removed by heavy alkaline degreasers, this is relatively ineffective and extremely aggressive to the underlying and surrounding surfaces. The normal method of removal involves a blend of volatile solvents combined with surfactants to ensure easy removal. Many people will use substitutes such as petrol/white spirits or even WD50 – whilst these are effective they can be difficult to remove after use, leaving residues which can be compromise later steps in the cleaning process and even cause long term damage to the paint.
A wide range of suitable solvents are available and the choice of these will typically come down to a balance between cost, safety and environmental considerations. The vast majority of inexpensive tar removers are based around non-renewable crude oil derivatives. With added cost, alternative solvents, such as citrus derivatives become viable giving the products a renewable character. The majority of these products are highly liquid and volatile and are prone to rapid evaporation and/or run off from the treated surfaces. This tends to result in a large rate of usage since multiple applications are often required, especially in warm environments. A limited few products address this, typically with the use of thickening agents in an attempt to increase dwell times and decrease product usage.
Iron contamination or fallout is a result of modern industrialization. Iron is introduced to the atmosphere from a wide range of industrial sources as well as other sources such as rail transport. This iron either settles or is carried onto the bodywork by rain where it can become bonded. Like tar, this contamination is insoluble in water and is thus not removed by regular washing routines.
Removal of metallic residues has been carried out routinely for many years in industrial environments. This typically has involved acidic products which will dissolve the contamination. These can range from the relatively safe (such as oxalic and phosphoric acids) through to the very strong (such as nitric and hydrofluoric acids) and the choice is often down to the materials being treated. Whilst these products are relatively inexpensive, they have the detractor that they will dissolve not only the iron contamination but will also act to corrode other metals when used repeatedly. A more recent class of products in this area is pH neutral fallout removers. These make use of a very specialized group of chemicals which bind to the iron and make it soluble in water. The starting chemical is colourless but the iron ‘complex’ which is formed is a distinctive purple colour. As a result, these products appear to bleed as they work. This is a natural consequence of the iron being dissolved and is not a trick produced by the addition of dye. Moreover, the same chemistry is used to quantify iron levels in some analytical techniques – more colour means more iron. As a result the more concentrated the bleeding fallout remover, the more it will bleed.