A coating is only as good as the preparation of the underlying surface. Surface preparation is the most important part of a coating system, because it affects the performance of the coating more than any other variable. If the surface preparation is poor, coating performance is likely to be poor even if an expensive coating is used. If surface preparation is good, then the coating applied over it is likely to perform well.
Professional blasters know that the key to achieving a lasting coating for any application is to choose the right abrasive depending on the surface being blasted and the surface profile desired. With the right abrasive, one can get the job done faster, minimise the volume of abrasive used and maximise the profits.
The following factors must be considered when choosing the abrasive.
As a general rule, we recommend that a blaster should use the finest abrasive available that is necessary to attain the characteristics of the required surface preparation. A finer abrasive will have more impact per unit of volume. The more particles in the stream, the more of the surface can be blasted at the same time. Many blasters make the error of using coarse abrasive when a finer abrasive can provide better results. For instance, instead of using a hard, expensive abrasive, or a coarse particle, crushed glass can be a good choice for work on relatively soft surfaces such as concrete or wood. A finer abrasive such as 80mesh garnet is usually cheaper than a coarse grade such as 30-60mesh.
Coatings adhere poorly to hard, flat iron and steel surfaces. Therefore, when preparing iron and steel for a protective coating system, the blaster is required to develop a pattern of indentations that the coating can anchor to, also known as the “anchor pattern” or “mechanical tooth”.
When a sufficiently hard abrasive particle strikes steel, it deforms the surface into a valley and pushes up peaks. The distance between the top of the peak and the bottom of the valley is known as the depth profile.
In the U.S., the depth profile is measured by mils – thousandths of an inch. In the metric system, the micron(one millionth of a meter) is used.
1 mil = 25.4 microns.
Generally, the correct profile depth will be 25-30% of the dry film thickness of the total coating system. For most industrial coatings, the typical steel profile is between 2-3 mils, not normally exceeding 5 mils.
The applied coating should completely fill the valleys and cover the peaks for optimal adhesion. The deeper the profile, the more anchoring occurs. However, if the profile is too deep, the peaks can protrude beyond the surface of the coating, causing pinpoint rust and premature failure of the coating. In some cases, the contractor may be required to go back over the area with a finer abrasive to reduce the surface to specified profile depth. This is a costly mistake and often results from a keenness to get the job done quicker using a coarser abrasive than necessary.
BLAST MEDIA PROPERTIES
The right abrasive can be chosen for the job once you know the surface characteristics and the profile depth that needs to be achieved. The following properties of abrasives contribute to profile depth.
In general, with all other factors remaining constant, the bigger the abrasive particle, the deeper indentation it will make. However, blasting large particles gives a lesser impact than an equivalent volume of smaller particles. Small particles clean faster, provide better coverage and result in a more uniform profile. The most efficient approach is to use the smallest abrasive particle necessary to achieve the desired profile.
Abrasive particle sizes are commonly classified by mesh sizes and are often given a range, for example: 30/60. The media particle distribution chart from the supplier will indicate the percentage of particles that fall within the range. For instance, a 30-60mesh chart may show that 95% of the mix will fall through a 30 mesh but not pass through the 60. The mesh size number indicates the number of mesh lines per square inch in a sieve, ranging from 6 (coarse) to 327 (powder).
Media can be angular, spherical or natural in shape and the ability of the media to cut depends to an extent on the shape of the granule. The shape of the article affects how deeply it cuts into the coating and underlying substrate. Angular granules generally have the best cutting ability and cut through soft coatings and rust, cleaning faster, and producing sharper anchor patterns. On the other hand, spherical shapes are useful in tempering or strengthening a part (through a process known as shot peening) and are suitable for breaking away hard brittle coatings and mill scale.
Spherical particles such as glass beads yield a semi-reflective sheen appearance whereas angular grit media (rock garnet, crushed glass etc) produce an etched matte finish that is characteristically bright but is non-reflective.
Generally speaking, the harder the particle, the deeper the profile it will impart, except in cases where a high-velocity hard particle shatters, delivering less than optimum force.
Hardness determines whether an abrasive particle can etch or provide an anchor pattern on a particular substrate. One way to determine the hardness of an abrasive is to use the Mohs’ hardness scale. The scale ranges from 1 to 10, with 1 being the softest (talc) and 10 being the hardest (diamond). Most abrasives that effectively achieve an anchor pattern on a surface have a Mohs’ hardness of at least 6.0.
Softer abrasives, like organic materials and plastics, are good for removing dirt, oil, grease and paint without creating an anchor pattern in the underlying substrate.
Dense particles impact with more kinetic energy over a smaller surface area, resulting in a deeper profile. Dense particles also deform less, absorbing less energy upon impact.
The density or specific gravity of an abrasive affects the cleaning rate and anchor profile produced into the substrate. Density is measured in pounds per cubic foot. Specific gravity is measured by the density of the particle-relative to the density of water. An abrasive with a high specific gravity is heavier. When propelled at the same pressure, a heavier abrasive achieves a deeper anchor pattern than that from a lighter one. Abrasives with a high specific gravity tend to be less dusty. Those with a low specific gravity impact the substrate with less force and are used for light cleaning, polishing, and deburring.
Friability refers to the propensity of the media to alter shape during the blasting process. In general, a high friability index will allow more reuse of the media but sometimes, repeated use may not yield a deep surface cut.
As you can expect, higher the speed, the greater is the impact. Similarly, abrasive propelled with higher force results in deeper profiles. Velocity is one factor that can be easily adjusted during the process of blasting. Velocity is generally a factor based on the pressure of the compressed air stream but is also affected by the density of the abrasive used. Once the right abrasive has been selected, the blast pressure can be adjusted to fine-tune the desired depth profile.
TYPES OF ABRASIVE MEDIA
The following are the common abrasives used in the sandblasting industry.
Garnet is a natural mineral with excellent naturally abrasive properties. This hard abrasive is fast-cutting, low-dust producing and low-consuming, excellent for removing tough coatings, paint, rust and mill scale from steel. Garnet permits precise feathering control. A good general outdoor surface preparation abrasive.
This abrasive is usually made from crushing bottle glass and creates a sharp profile and is useful in removing a variety of coatings. It produces a whiter, cleaner finish than slags and mineral sands. Crushed glass is the popular choice of abrasive for preparing concrete surfaces.
Mineral sands, like olivine and staurolite, are fast-cutting, low dust abrasives that contain less than 5% silica per volume. They are useful for removal of rust, paint, weathered coatings and mill scale.
Coal slag is considered a “green” abrasive because it is a by-product of coal-burning power plants and is otherwise disposed of as waste. It is a relatively cheap, low dusting abrasive with low free silica, but is considered a dirty abrasive and not widely used in wet abrasive blasting because the high proportion of fine particles that mud up on the surface. Typical applications include the removal of rust, paint, weathered coatings and scale from steel and concrete.
Glass beads are used for general cleaning, peening and cosmetic finishing of sensitive metal surfaces; removing automotive paint; brightening grout and removing fungus and calcium deposits from tile; polishing cast iron, stainless steel, aluminium, propellers and turbine blades.
Plastic is a soft, light abrasive that leaves no anchor pattern. It is sometimes used for stripping paint and mould from sensitive surfaces, deburring and de-flashing aluminium, brass, plastics and fiberglass. It is considered a less-hazardous alternative to chemical stripping and faster than hand-stripping.
Nut shells and other organic materials such as corn cob do not create anchor patterns, making them useful for cleaning dirt, grease, oil, carbon, scale, burrs and paint without changing the underlying substrate. They are natural by products and depending on availability are suitable for cleaning auto body panels, electric motors and aircraft engines, dies and moulds, polishing watches, jewellery and restoring antique surfaces.
Silica sand is the original blasting abrasive but it is no longer recommended for blasting applications due to the occupational health hazard of silicosis.
The right abrasive for the job is the finest grade that can impart the depth profile required by the coating system. Before purchasing abrasive, visit the manufacturer’s websites for the latest product specs, or browse our directory of abrasive manufacturers product data sheets.
The following table summarises the characteristics of different types of media. The data is an approximations of general product categories and specific products can vary significantly. Check the manufacturer’s data sheets for the most current and accurate information
It is important to note that media cost is only one of the many costs of blasting. Refer to the article on Total cost of Blasting to understand all the variants and the interplay of the factors that determine the true total cost of blasting.