ABRASIVE BLASTING IN PRACTICE
Copyright 1993/2001 Mario S Pennisi
Email: mariopen@coatfab.com

WHY IS SURFACE PREPARATION IMPORTANT?
Surface preparation is the most important part of a coating system, because it affects the performance of the coating more than any other variable. Given that the correct coating system is selected, if the surface preparation is poor, coating performance is usually going to be poor. If surface preparation is good, then the coating applied over it is likely to perform well.

It is useful to know the reasons why surface preparation is so important, because knowing why can help the applicator do a better job.

SURFACE PREPARATION AS A FOUNDATION
Surface preparation is to a coating system what a foundation is to a building. If a building has a poor foundation, it can list or lean, as the famous Leaning Tower of Pisa does, or it can collapse altogether. If a coating system has a poor foundation (surface preparation) it will fail sooner than expected (say, after five years rather than 10 years); or it can fail catastrophically within the first year of application. In both instances great financial losses can occur to a facility owner. Surface preparation creates a foundation in two important ways:
* a mechanical way, by providing an anchor for the coating; and
* a chemical way, by allowing intimate contact of coating material molecules with the steel (or other material) surface.

WHY AN ANCHOR PATTERN?
When a surface is very smooth, coatings have a difficult time adhering strongly. A scraper or even a fingernail, for instance, easily removes a coating on glass. On the other hand it is difficult to remove a coating on a rough surface like sandpaper. Steel, when it is abrasive blasted, has a surface that is rough like sandpaper, with a series of tiny peaks and valleys called surface profile. Coatings anchor themselves to the valleys of the profile, and the peaks are like teeth. This is why surface profile created by blasting is sometimes called an "anchor pattern" or "mechanical tooth."

SURFACE SOILS
- Visible contaminants
Soils on a metal surface are made up of many different materials. They include:
* Oil
* Grease
* Corrosion products
* Oxides/Mill Scale
* Perspiration
* Marking pen ink
* Rubber from boots
* Brazing flux
* Weld flux
* Weld scale
* Weld anti-spatter compound
* Adhesive from tape
* Dirt/dust
* Chemicals/Salts
* Smut
* Metal chips
* Drawing compounds
* Polishing/buffing compounds
* Abrasive
* Mould release agents
* Previous organic coatings
* Previous metallic coatings
* Finger prints
* Silicone.

When contaminants such as these are painted, they interfere with the mechanical and chemical adhesion of the coating to the substrate so that the coating is likely to fail. On the other hand, when all soils are removed, the coating can achieve complete and continuous contact with the substrate, thus assuring the best possible adhesion. When a coating adheres well, it is likely to be an effective barrier. The coating can minimise or prevent moisture (the electrolyte in the corrosion process) from reaching the substrate.

- Non-visible contaminants
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.

.....Abrasive Blasting Media -Garnet .....................Glass Media

* Abrasive blasting
There are four (4) levels of abrasive blasting described in AS1627-Part 4. The following lists them in increasing levels of cleanliness:
- Class 1
- Class 2
- Class 2 1/2
- Class 3.
These specifications define the physical cleanliness that must be achieved on the surface.
- Cleanliness The last three of these specifications requires all the mill scale, rust, and old paints to be removed. All that can remain on the surface are stains of these contaminants.

The following amount of stains is allowed (by visual estimation).
Class 2 - 33 percent of each sqcm,
Class 2 1/2 - 5 percent of each sqcm
Class 3 - none.
The most common tools used in the industry to assist in determining surface cleanliness are
SSPC - Vis 1-89,
NACE coupons, and
Swedish pictorial standards as specified in AS1627.9

- 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.

ASSURING QUALITY OF ABRASIVE BLASTING OPERATIONS
The performance of a coating depends in large part on the quality of surface preparation. This is because coatings have been formulated to perform properly under particular conditions, such as over a specified degree of surface cleanliness and a specified anchor profile, and under certain environmental conditions. If these and other conditions are not met, coatings may not achieve their expected performance. When dry abrasive blasting is the specified method of surface preparation, many conditions must be taken into consideration by the blaster.
* Checking conditions before blasting
Most coatings do not adhere well to surfaces contaminated with oil and grease. Blasting actually drives them further into the steel rather than removing these contaminants and thus contributes to premature coating failure. Therefore, a check for visual surface contaminants before blasting is essential. If oil and grease are present, they should be removed with solvent cleaning, in accordance in AS1627-Part 1.
All of the blast cleaning specifications from AS1627-Part 4 require this step with the statement, Before blast cleaning, remove visible deposits of oil or grease by any of the methods specified in AS 1627 Part 1.

Ambient conditions should be measured before blasting. If blasting is not to be followed immediately by coating application, then it may be all right to proceed first with rough blasting to remove the existing coating, rust, and mill scale, and to check ambient conditions before the final blast. If blasting is to be followed immediately by coating, then ambient conditions should be checked before blasting begins.

It is essential that the dew point, air temperature, relative humidity, and surface temperature are suitable for blasting. This insures that condensation will not be forming on the metal surface during or after blasting and cause flash oxidation (rust), which can be detrimental to the overall quality and coating performance. Dew point is the temperature at which moisture condenses on a surface. If the dew point is 10C, condensation will occur if the metal is at or below this temperature. As a general rule, final blast cleaning should take place only when the surface is a least 3C above the dew point. For example, if the dew point is 10C, the steel temperature should be at least 13C. This rule provides a margin of error, in case of instrument inaccuracies, quickly changing weather conditions, or human error. Dew point is calculated using the relevant psychrometric tables. The psychrometer is a hand-operated instrument that has 2 glass thermometers.

To measure ambient conditions, the following equipment is required:
- a surface temperature gauge or surface thermometer.
- a psychrometer for measuring dry bulb (air) and wet bulb temperature, and psychrometric tables for calculating dew point and relative humidity.

* Checking blasting abrasives and equipment
Abrasives and equipment should also be checked for cleanliness before blasting, and the equipment should be checked for efficiency. There are several parts of the blasting equipment that need to be checked for contaminants:
- the compressor,
- the moisture separator, and
- the air that comes through the hoses.

* Checking the surface after blasting
After blasting, remove all dust from the blast-cleaned surface, either by blowing down the surface with compressed air or by vacuuming. Dust on the surface can interfere with the coating's ability to bond to the surface. After blowing or vacuuming the surface, brush a clean white cloth across the surface (without touching the substrate with the hands as body oils or salts can be transferred easily to the surface and contaminate it). If dust appears on the cloth, blow down or vacuum the surface again. Check for non-visible contaminants, especially soluble salts, which are detrimental to coating performance. Once the blasted surface is free of dust (and other contaminants), check the surface profile and degree of cleanliness to see that the specifications have been met.

RECORD KEEPING
The quality control checks should be documented and kept as part of the quality control records for the job. This way, historical information is available for verifying compliance with specifications.

CONCLUSION
There's an old cliche that a top quality coating put on a marginally prepared surface will perform no better than a cheap coating applied to a squeaky clean surface. This statement is probably an oversimplification of the problem, but there's a great deal of truth in the statement as well.
Surface preparation is an important step that affects the life of a coating. The life of an oil-based paint, for example, is longer on a blast-cleaned surface than on a hand-cleaned surface.
Many of the high technology coatings such as zinc-rich primers require a blast-cleaned surface to stick and provide protection to the steel. Coatings stick better to a rough surface than to a smooth surface.
Abrasive blasting both cleans a surface and roughens it. This roughness is called anchor profile. The specification will specify how rough the surface must be before the paint is applied.
Quality assurance will help ensure that abrasive blasting operations create a surface suitable for coating application and should be followed even if customer's inspectors are on the job conducting similar checks.