Blast Cleaning is the process of treating a surface by propelling particles at high velocity towards it. This can have a range of applications from patio cleaning to artificially ageing stonework. Small power washers can be purchased or hired relatively cheaply for small jobs such as cleaning walls or stonework, although they may not be suitable for some types of organic staining. This will require blasting with grit or a similar substance, although this is likely to remove layers of the material you are trying to clean as well as the dirt. If it is important for the surface to be preserved a chemical treatment may be more suitable.
The two main factors that can be varied in blast cleaning are the type of particle that is “blasted” and the pressure at which this is done.
Pressure: The pressure can normally be adjusted during the job to ensure that only the desired layers of material are removed. Using a overly high pressure is inefficient in that more energy is used than is necessary and more particles will be used increasing cost and the amount of waste. A small test area, ideally somewhere discrete, is chosen and used to find the exact pressure required for the job. If the surface that is to be cleaned or treated is not uniform, the pressure may need to be adjusted as the work is carried out. This is a difficult task and requires a highly skilled and experienced person.
Particle size and type: grit, water, co2 etc. – also have implications on cleanup.
- Grit is the most traditional substrate 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 aluminium.
- Calcium Carbonate is normally used at low pressure to remove grime from delicate surfaces such as fabric or wood. Interior beams that needed revitalising, 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 when other decoration work is taking place. Ask the company about the best order to carry out work as doing things in the wrong order could lead to repeating tasks and increasing costs. The effectiveness can be increased at low pressures by using a vortex pressure system. The most common use is to strip paint from aluminium 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. 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 mould 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 mould removal and are particularly effective because of their decarbonising 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.