C1 - Proper stand-off distance

To achieve the best blast pattern and profile on a surface while maintaining productivity, the abrasive nozzle should be kept at an appropriate distance from the work surface. 

Being too close to the surface will produce a narrow blast pattern. Hence, it will take longer to prepare the area. 

Holding the nozzle too far from the surface can result in poorer cleaning and the required profile might not be obtained. 

For garnet media, the stand-off distance is ideally 46cm (18in) (although this can depend on the type of nozzle used).

C2 - Blasting angle

The angle of blasting can affect the productivity depending on the original state of the surface. 

In general, a 90 degree angle will cause the abrasive to rebound, leading to reduced performance. However, in extreme cases, such as heavily rusted or pitted surfaces, angles between 80 and 90 degrees may prove to be most effective. 

Too narrow an angle could cause the abrasive to skim across the surface, resulting in reduced profile.

When removing old paint, 45 to 60 degrees is typically used and for general cleaning it is 60 to 70 degrees. 

When blasting with garnet, the most effective angles tend to be between 55 and 70 degrees, depending on the nature of the substrate.

C3 - Blasting style

The time spent cleaning any particular spot or area will vary depending on the operation. This can be very short when removing loose materials, but will be considerable longer for tightly adherent material. 

The blast operator should use a steady movement and not dwell unnecessarily on any particular area. 

To ensure consistent levels of cleanliness and surface profile, an overlapping sweeping motion should be utilised.

The blasted surface profile should be checked to ensure that it complies with the relevant specification. 

This can be done by utilising Testex Tape in conjunction with a spring micrometer, or by the use of a digital profile meter.

Blast operatives should have knowledge and experience of best practice and techniques, whilst also understanding the value of good surface preparation. 

Most importantly however, they should be properly trained.

• Maintain the correct distance and blast angle.
• Use an even, overlapping sweeping action.
• Do not dwell too long on any one area.
• Check the surface profile meets the requirements of the specification.
• Ensure that operators are properly trained.

   By John Halewood, GMA Group