The process known as shot blasting can be used to both roughen a surface or to smooth it. The aim is to remove old coats of paint, rust and contaminants. It can also reveal any faults in the material such as cracks or broken welds that have come to the surface and may need to be fixed. The process can be used to clean concrete, stone, brickwork metal or hardwood. The resulting smooth or rough surface is varnished, painted, coated, or sanded depending on the final product. All oil and grease should be removed from the surfaces prior to blasting.
A variety of materials can be used during the blasting process. Sand is the most common abrasive. It is used to remove pollution, weathering and graffiti from stone, concrete and brickwork. It removes heavy rust from metal surfaces on vehicle and industrial machinery bodywork and can stop the rust from penetrating the surface. The same process may create an antique effect on hardwood such as oak in oak building frames, flooring and garden furniture.
Garnet is a hard abrasive used in the blasting process for the preparation of non-ferrous metal surfaces. Chilled iron granules are cheaper to use than garnet for the same uses but should not be applied to any surface that is exposed to moisture. Olivine, a dark green mineral, which can also be of gem quality, can be used as an abrasive on decorative stone and nonferrous metals. Glass beads are more economic abrasives used more often for polishing rather than the removal of surface coatings.
There are two ways of propelling the abrasive through a blasting machine. A compressed air system can be fitted to a hand held rubber hose and nozzle. It can deliver the shot by pulling a trigger. This system is best for removing paint and rust from complex metal surfaces. It may be angled to hit the surface at the best direction for cleaning.
A more common and cheaper system form blasting is a centrifugal turbine. This can also deliver shot at a chosen speed, direction and quantity. The cleaning power depends on the capacity of the turbine’s motor.
Once the surface is clean and has the required texture, a number of coats can be applied. A wet spray can cover the surface of a metal with rust retardant that can be a primer for any later top coat of enamel or paint. Another possibility is a spray of fine electrically charged powder the particles of which fuse together on the clean surface to create a texture of choice. If the aim is to preserve the natural colour of the surface material, then it may just be lacquered after the blasting is completed.
This guest blog post was written by Jane Ransome, a consultant for abrasive blasting cleaning operations for buildings and artefacts, who writes for www.sfeg.co.uk.