Abrasive Blast Cleaning: Part Two

Abrasive blast cleaning is the most cost effective process for preparation of a surface prior to painting and is usually used where long-term protection is required. Blast cleaning can be carried out manually, or by using automated blast cabinets and the pressure can be varied to suit the type of metal or treatment required. Specialized materials or blast media in a variety of sizes or grades are used for treating different types of metals, surfaces and components, depending on the finish required. See Figure 1 below for the most commonly used materials in the blast cleaning process.

Figure 1: The most commonly used materials in the blast cleaning process

Particle size and type: grit, sand, water, co2 etc. – also have implications on cleanup.

• Grit and Sand are the most traditional substrates used for blast cleaning. There are a wide range of different types however, each with different applications. The cheapest standard grit is suitable for cleaning heavily corroded steel, marine growth and cement build-up. It would also be used before a metal media is used to treat a surface.

More expensive grits create a finer etch on the surface and give a more efficient clean in terms of time and amount of substrate required. More coarse grits with higher content of stone particles are used to remove thick layers of grime and oil from stone surfaces. A coarse grit will also leave an authentic aged effect on older stone once cleaned.

A finer media would give an uncharacteristic effect to the stone and probably remove any charm and aesthetic value of old walling. A finer grit would normally be employed for use upon surfaces with a special finish such as stainless steel or aluminum.

• Calcium Carbonate is normally used at low pressure to remove grime from delicate surfaces such as fabric or wood. Interior wooden beams that need revitalizing, for example, could be treated in this way. Any work that is to be carried out inside will not be a small operation owing to the clean-up costs and so it may be advisable to carry this out after other decoration work has taking place. The effectiveness can be increased at low pressures by using a vortex pressure system. The most common use is to strip paint from aluminum surfaces.

• Dry ice is becoming an increasingly popular choice for blast cleaning owing to the fact that it evaporates and therefore the cleanup costs are massively reduced. It has distinctly different properties to sand blasting, however, and therefore is not directly comparable. Much of the effectiveness of using dry ice is down to the sudden cooling effect on the surface that can be used to separate a coat of resin from metal for example. The particles of dry ice expand as they sublime from solid to gas. This means that any cracks in the surface will be forced open by this action. Carbon dioxide is non-conductive and can be used without any risk of interference with electrical equipment or safety risk to the user.

• Air or water – Non contact cleaning using air or water is not "blast cleaning" as such, but employs the same principle to remove debris or waste or to clean surfaces. Small air blowing guns can be very useful for intricate cleaning jobs such as removing mold from tiling grout in bathrooms.

• Specialist – There are some more unusual substances that can be used for particular processes to create effects. Walnut shells are used for polishing and mold removal and are particularly effective because of their decarbonizing action. Ferro silicon is a very aggressive choice and can also be used to harden surfaces or making them non slip as a residue of silicon remains on the surface.