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OVERVIEW

Many blasting and coating specialists use, by default, a 30/60 or coarser mesh garnet for surface cleaning and preparation. But a more efficient — and often overlooked — solution is to select the specific garnet grain size known to achieve the desired surface profile. The size of the abrasive particles can significantly influence surface cleaning rates, as the number of grains in standard garnet grades can vary from 5 to 15 million per pound. 

This article outlines the advantages of finer- versus coarser-grade garnet on industrial coatings, mill scale, lightly rusted surfaces and light-build coatings. It also provides examples of industry applications that benefit from the use of finer-grade garnet and, for comparison, applications in which the use of coarser grades are more appropriate. Further, the author explores practical considerations for choosing the garnet grain size based on the desired surface profile.

BENEFIT OF USING FINER ABRASIVE

When blasting with garnet, larger garnet grains facilitate deeper indentations than finer garnet grains but produce fewer impacts across any given area. This is because fewer grains per square inch are hitting the surface area. The use of a finer grade with a higher grain count per pound will produce more impacts per square inch, resulting in more efficient blasting performance on lightly rusted coatings, mill scale and heavy corrosion. 

In addition to the benefit of higher blasting rates, finer garnet grains produce a more uniform surface profile, cleaner finish, higher peak density and are able to sweep out rust within tight crevices. Figures 1 and 2, below, demonstrate the difference in impact produced by coarser versus finer garnet grains, and the resulting surface finish, on a coated metal.

Figure 1. Coarser garnet grains

 

Figure 2. Finer garnet grains

SELECTING THE RIGHT GARNET ABRASIVE

Whatever the required finish, there will be an appropriate garnet grain size for the job. For general applications when a surface profile above 75 microns is specified, the 30/60 mesh garnet is generally considered to be the ideal industry standard garnet grade. Finer garnet grades range from 80 to 120 mesh size, and ultra-fine garnet grades range from 200 to 350 mesh size. 

When selecting a garnet grade, the most important criteria involve the physical nature of the required finish (e.g., surface profile and standard of cleanliness). For example, when surface profiles above 75 microns are required, a relatively coarse garnet may be appropriate; however, most of the time, the purpose of blast-cleaning is not just to attack the surface aggressively. The operator should aim to achieve the required surface profile without unduly exceeding the surface preparation requirement. 

Finer-grain garnet has the versatility to achieve the desired results for specialized applications by reducing the potential for undue damage to work pieces. The use of finer grains at the appropriate nozzle pressure will facilitate control and efficiency. This principle applies to delicate substrates such as fiberglass and aluminum. 

APPLICATION EXAMPLES FOR FINER-GRAIN GARNET

Sweep Blasting

This process is often used to prepare structures where an existing coating has deteriorated or a recently blasted surface has flash rusted. Intact paint work needs to be brush-blasted to remove loose paint and contamination and produce a surface profile for subsequent recoating. Any corroded areas will also need to be blast-cleaned to a specified standard. A smaller-grain garnet abrasive will effectively etch the intact coating while scouring into any existing surface profile on the exposed substrate, enabling the original profile to be efficiently re-serviced for the new coating. It also facilitates a feathered transition between intact coating and bare steel.

General Maintenance

Blasting with a finer-grade garnet is beneficial when you have low-build coatings or heavily pitted surfaces. Using a finer-grade garnet on low-build coating systems results in a faster blast with more particles per square foot impacting the surface. In the case of heavily pitted surfaces, finer garnet particles are able to fit into pitted areas to clean where coarser particles cannot.

Due to the low dusting characteristic of garnet abrasives, surrounding areas are not impeded by undue contamination. This is an additional benefit, resulting in more efficient surface preparation throughout the duration of the project. 

Architectural Concrete

In order to achieve a quality, uniform concrete finish, as specified, it is essential to use a finer-grade garnet. Often, using a coarser abrasive can result in over-blasting, which can produce an unacceptable surface finish. Once that happens, there is no going back.

Aluminum Structures and Galvanized Surfaces

The scouring effect of smaller grains can serve to clean away contamination from surface undulations. Using lower air pressure will avoid undue damage to relatively soft surfaces.

Fiberglass

Repairs to fiberglass surfaces often require removal of top layers of resin without causing damage to the gel coat. Using smaller garnet grains will facilitate controlled surface preparation, allow for localized repairs and create the desired feathering effect on the intact resin.

Graffiti Removal

Typically, the removal of graffiti will need to be repeated on regular occasions. Hence, the facility to do the job without undue damage to the substrate is invaluable. The use of a finer-grain garnet gives the blaster greater control and helps wear away unwanted decoration without ripping into the surface.

Turbines and Propellers

Even the finest grade of garnet remains a hard and sharp tool. When turbines and propellers become encrusted with detritus, ultra-fine garnet at controlled pressures can be used for brush-blasting without affecting the dynamics of the work piece.

CASES WHEN FINER GARNET GRAINS MAY NOT BE APPROPRIATE

Creating a variety of finishes in accordance with specific requirements can be achieved by combining different garnet grades at variable pressures. Ultimately, abrasive performance is determined by energy resulting from the particles' velocity, size and density (E = ½ mv²).

When removing thick, soft coatings, the energy may be absorbed before the garnet grains achieve full impact. Hence, the heavier features of larger-grain garnet can be more effective. Equally, when blasting off high-build coatings above 500 microns in thickness and heavy corrosion on ship hulls, finer garnet grains may not be appropriate.

PRACTICAL CONSIDERATIONS

• Optimal blasting performance is achieved by matching the correct garnet grain size to the specific job, with clean, dry compressed air at the appropriate pressure in conjunction with controlled abrasive flow rates. 
• This ensures a uniform surface profile without rogue peaks or oversized troughs, facilitating efficient paint application.
• The unique physical nature of garnet — sharp, hard and tough — allows usage at lower air pressures, which avoids over-blasting and unnecessary damage to the substrate.

CONCLUSION

The physical nature of finer-grade garnet facilitates better blasting performance. Finer garnet grades can enable more controlled blasting, more efficient blasting performance and cleaner surface profiles up to 75 microns. Certain surface coatings, such as mill scale, light rust and low-build coatings, can be removed more effectively with finer- rather than coarser-grade garnet. Specialized finishes on applications like graffiti removal, fiberglass and flash-blasting are made possible by the combination of finer grains and variable air pressure. Selecting the smallest particle size required to achieve the desired result will reduce the need to paint over rogue peaks or fill oversized troughs within the surface profile. 

To obtain the right garnet abrasive for the desired profile, it is important to consult a garnet expert during the planning stage of a project. This can result in increased blasting rates of more than 10 percent and decreased abrasive use by up to 10 percent, driving down project times and cost.