A primer on surface preparation

Silica sand is an inexpensive abrasive that is readily available in many areas of the country. It ranges from 6 to 7 on the Mohs hardness scale and provides an etch or anchor pattern on a steel substrate. Available in many sizes, silica sand is 95 to 99 percent silicon dioxide. However, special precautions must be taken when blasting with silica sand because of the silicosis hazard it presents.*

Coal slag is a by-product of the utility industry. This low-cost abrasive is blocky in shape and contains less than 1 percent free silica. Coal slag has a Mohs hardness of 6 and provides an etch or anchor pattern on a steel substrate. In general, it is a one-time-use abrasive.

A by-product of the copper ore industry, copper slag is blocky in shape and contains less than 1 percent free silica. It has a Mohs hardness of 6 and provides an etch or anchor pattern on a steel substrate. Because of its high specific gravity, copper slag has a tendency to embed in a steel surface. Available in many sizes, it is considered a low-cost abrasive.

A naturally occurring mineral, staurolite is angular in shape and contains less than 1 percent free silica. It has a Mohs hardness of 7 and provides an etch or anchor pattern on a steel substrate. Staurolitemay be recycled several times. It is available in medium to fine sizes.

A manufactured abrasive, ceramic is angular in shape and contains less than 1 percent free silica. It has a Mohs hardness of 7. Ceramic is available in various sizes and will provide an etch or anchor pattern on a steel surface. It may be reclaimed several times.

Aluminum oxide is a fast cutting, low dusting, higher cost abrasive used in blast cabinets and rooms with reclaim systems. This manufactured abrasive is angular in shape, contains less than 1 percent free silica, and has a Mohs hardness of 8. It will provide an etch or anchor pattern on a steel surface. Aluminum oxide may be reclaimed many times.

Glass beads are a fast-cleaning, higher cost abrasive used in blast cabinets and rooms with reclaim systems. Glass beads can be reclaimed many times. They are also used for surface hardening and deburring. This manufactured abrasive has a Mohs hardness of 6. Glass beads are available in many sizes and will peen a steel surface. Glass bead blasting leaves a matte finish.

A man-made abrasive, steel shot is spherical and contains less than 1 percent free silica. Its hardness, measured in Rockwell C, is generally 42 to 50. Steel shot may be recycled more than 100 times, which reduces its application costs. It is primarily used in rotary wheel blast machines and for enhancing surface hardness on various metals.

Steel grit is a man-made abrasive that is angular in shape and contains less than 1 percent free silica. It can be manufactured to various hardness levels ranging from 42 to 62. Steel grit may be recycled more than 100 times. It is primarily used in blast cabinets and rooms with recovery systems. It is also added to steel shot in many rotary wheel blast machines to help achieve a specified anchor profile.

* National Institute for Occupational Safety and Health Recommended Standard PB-246-697 states: “Uncontrolled abrasive blasting with silica sand is such a severe silicosis hazard that special attention must be given to this problem. Silica sand or other materials containing more than 1 percent free silica, should be prohibited as an abrasive substance in abrasive blast cleaning operations...The most certain and direct method of eliminating the silicosis hazard is to substitute less toxic material for free silica. In abrasive blasting, where silica sand can be replaced with an abrasive containing less than 1 percent free silica, such substitution should be considered.”