Other forms of soils, not always visible to the naked eye, are chemical contaminants. The most dangerous forms of chemical contaminants are soluble salts:
chlorides and sulphates.

When such contaminants are painted over, they have the power to draw moisture through the coating (osmosis) to cause blistering, detachment, and accelerated corrosion of the underlying metal. When structural steel is repainted, rough or pitted areas visible after dry abrasive blast cleaning may contain soluble salt contamination, especially in the base of the pits. Dry abrasive blasting does not remove these salts. It is wise to check for the presence of soluble salts with specially designed field test kits before painting and if they are present in detrimental amounts, to take additional cleaning steps to remove the salts.

Degrees Of Surface Preparation
In any job specification, the degree of cleaning required for a given substrate before painting depends on a number of factors.

* The service environment of the coating system. This is perhaps the most important factor, and normally is the first consideration when determining the degree of surface preparation. Generally, the more severe the environment, the better the surface preparation required. Severe service environments include:

- immersion in liquids,
- exposure to aggressive chemicals or environments, and
- high temperatures, or combinations of these conditions.

* Another consideration is the generic kind of coating used. Some coatings, such as alkyds, because they flow out and wet the surface well, can tolerate application over minimally prepared or hand-cleaned surfaces. In addition, some epoxy mastics and other "surface-tolerant" coatings are formulated to be applied over hand-and power tool-cleaned surfaces. Coatings such as vinyls and inorganic zincs, however, are at the other end of the spectrum. They require a higher degree of cleaning than many other types.

* Cost is another factor when selecting the degree of surface preparation. Blast cleaning to Class 3 (White Metal) is about 4-5 times more costly than to Class 1 (Light Blast Cleaning). In some severe environments and with some coating types, rigorous cleaning is necessary; but in other instances, cost and cost-benefit in the form of longer coating lifetime will become an important factor in selecting the degree of surface preparation.

* Finally, regulations may have an impact on the degree and method of surface preparation. - in residential or congested urban environments, open blasting may be prohibited and - in addition, where lead-or chromate-based paints are being removed, hazardous waste regulations may require containment and use of special surface preparation methods.

Determining the degree of surface preparation, as described above, is the job of a specifier or engineer. The task of doing the work is the contractors. No matter what degree of surface preparation is required, it must be done thoroughly. If hand-tool

cleaning is required, then all the surface area specified must be hand-tool cleaned, after it has been cleaned by water or solvent to remove dirt, oil, or grease. Similarly, if a Class 3 blast is specified, then conformance with the written description of this must be achieved on all surfaces. When preparing metal, it is also important to follow the proper sequence.

* First, remove dirt and other soils. It is a lot easier to sweep mounds of dirt and other loose material off a surface with a broom than to try to remove it with surface preparation tools.

* The next step is removing visible oil and grease by solvent cleaning. Then conduct the mechanical cleaning operation whether hand tool, power tool, or blast cleaning. If these steps are reversed, particularly with blast cleaning, the force of the blasting abrasive can drive the soils into the roughened steel surface or profile. Then it is not easy to remove, and it may interfere with coating adhesion. In addition, it is important to achieve the surface profile required by the specifications, because:

- When the profile is too rough, the coating may not cover the peaks of the profile, and the result will be pinpoint rusting.

- When the profile is not rough enough, the coating may not anchor well to the surface, and the result will be loss of adhesion. To make sure that a coating system will perform well as a barrier to prevent/reduce corrosion, all soils must be removed so that the coating contacts the entire surface of the metal for chemical adhesion and that the surface is roughened for mechanical adhesion as well. These two conditions of cleanliness and profile ensure that a proper foundation has been created for applying the coating system. This good foundation should help to provide many years of service life for the coating.

How Clean Is Clean?
Protective coatings have been used for centuries to protect substrates subjected to the environment. The most severe environments are near the ocean. Many industrial plants have severe exposures that require protection.

For years, most coatings were applied by brush to hand-cleaned surfaces. The fish oils and long and medium oil alkyd coatings were pigmented with some very effective inhibiting pigments such as red lead, and they were literally scrubbed into the surface by the brushing action. These materials were good at sticking on the surface, and they could be applied over mill scale and tight rust to provide some protection.

Of course, they would protect longer if they were put on a blast-cleaned surface. Many of the new, high performance coatings do not stick well to mill scale, tight rust, or a smooth surface. Abrasive blasting is needed to clean and roughen the surface.

Anchor profile The height of the anchor profile is specified independently from cleanliness. The manufacturer's application data sheet will give this information. There is no standard anchor profile height that is good for all coatings. The surface must be roughened sufficiently to get the coating to stick. Coatings that are applied in thin coats, such as oil-based coatings, require a low anchor profile ie, 50 microns. Too heavy an anchor profile will result in the peaks of the profile in the steel sticking out and causing pinpoint rusting. Thick coatings such as coal tar epoxies require a deep anchor profile ie, 100-125 microns, to get them to stick properly. Two common tools are used to determine anchor profile:

• a surface profile comparator and
• replica tape.