Some of the most commonly used methods are:
- Hand tools
- Power tools
- Abrasive blasting
- Centrifugal Wheel Blasting
- Water Blasting
To achieve the different surface preparation qualities, numerous methods may be employed
1 Hand tools
Hand tools Preparing surfaces using hand tools is one of the oldest methods known. It is generally used when power tools or other type of cleaning equipment cannot be used or are not available. It is also used when the areas to be cleaned are fairly small or inaccessible. Hand tool cleaning is designed to remove only loose rust, loose mill scale, loose paint and any other loose contaminants or detrimental foreign matter. It is not intended to remove right rust, mill scale, paint, etc. Tools normally used in hand cleaning include wire brushes, scrapers, chisels, chipping hammers, knives, abrasive pads or any type of non-powered tool that achieves the desired cleaning quality. Hand tool cleaning can be slow and therefore expensive given the manpower required. Deep marks, burrs, etc. are often left on the surface from tool impact and can e also normally required when using hand tool cleaning interfere with coating performance. Special surface tolerant coatings are also normally required when using hand tool cleaning.
2 Power tools
Power tool cleaning is intended to remove loose rust, mill scale, paint, and such materials and most of them are not designed to address tightly adhering rust, mill scale, paint, etc.
Like hand tool cleaning, it can also be slow due to high equipment wear, high worker fatigue and high manpower requirements. Power tools normally used include chipping and scaling hammers, needle guns, bumble bees, etc., all of which are piston driven. Rotary type tools such as abrasive discs and flap wheels are also used. Power wire brushes may also be used but are not recommended as they tend to burnish a surface which limits the mechanism bond between the coating and surface.
Special surface tolerant coatings are normally required when coating surfaces are prepared using power tools.
3 Abrasive blasting
Surfaces are cleaned using pressurized equipment that meters abrasive particles into a stream of compressed air conveying the particles through a “blast hose” and, finally, a “blast nozzle” onto the surface to be cleaned.
The end effect of this process is the removal of mill scale, rust paint and other detrimental contaminants by the blasting action of these abrasive particles to the quality of cleaning specified.
Sand and / or grit blasting equipment is usually very portable and available at reasonable costs throughout the world.
Two drawbacks of this type of surface preparation, however, are its limitation to outdoor use due to dust and abrasive fallout and its susceptibility to weather changes.
Dust and fallout can be limited somewhat by using “wet blasting” equipment. This equipment wets the abrasive flow with water, thereby reducing dust levels.
Rust inhibitors or surface tolerant coatings are normally required when using this type of equipment
4 Centrifugal Wheel Blasting
This method was first introduced in 1932 and is mostly used in shop environments, although several types of portable
units are now available for onsite use. Basically, this method uses motor driven bladed wheels that throw abrasive
particles by centrifugal force. These wheels are available in several sizes and are powered by electric motors of up to 100 horsepower. Most machines have at least four wheels, which are positioned so that the abrasive particles reach the entire surface to be cleaned.
The material to be cleaned is passed through an enclosed cabinet where the wheels are located using a conveyor system. The abrasive is thrown against the material, then dropped into hoppers located beneath the blast area and is sent to the abrasive reclaiming system for reuse. Rates and quality of cleaning can be varied by changing the number of wheels used, speed of material past the wheels and type of abrasive used. This method offers considerable savings in time, labor, energy and abrasive consumption versus abrasive air blasting. For example, a four wheel, A 30 horsepower wheel unit will throw about 3,200 pounds of abrasive particle per minute. A “sandblast” operation would require 44 3/8 inch diameter nozzles and a 3,00 horsepower compressor to equal that abrasive capacity.
Another pattern will also stay constant due to the automated system.
One extremely important point to be observed in this method is the requirement of pre-cleaning all materials to be blasted per SSPC-SP1 to ensure that all oil that oil and grease are removed. If this is not done, or not done properly, the recycled abrasives will become contaminated and will contaminate contaminate each part going through the cabinet and increasing chances for premature coating failure.
5 Water blasting
Water Blasting – This method began in the early 1950’s and is being employed regularly today. There are several configuration by which water blasting can be performed:
- Low Pressure Water Washing – pressures less than 350 bar (5,000 psi)
- High Pressure Water Cleaning – pressures of 350 – 700 bar (5,000 to 10,000 psi)
- Water Jetting – pressures above 700 bar (10,000 psi)
- High Pressure Water Jetting – pressures of 700 – 1700 bar(10,000 to 25,000 psi)
- Ultra-High Pressure Water Jetting – pressures greater than1700 bar (25,000 psi)
Usually anything that can be removed by hand tool, powertool or abrasive blast cleaning can be removed with a stream of high pressure water. While its rate of cleaning is faster than hand and power tool, it does not always remove tightly adhered coatings. It uses the cheapest abrasive available – water, and is especially useful for maintenance surface preparation due to the lack of sand and grit that may get into equipment.
Sand / grit injection units or attachments are available that will assist in the removal of extremely tight coatings and produce surface profiles. Rust inhibitors and/or surface tolerant coatings are normally required when using this method of cleaning. Worker fatigue can be high in this method due to back pressure developed at the nozzle. Newly developed equipment has reduced this factor.
The high pressure involved in this method also requires following good safety practices. The foregoing is a brief overview of the most commonly used methods for preparing a surface for coating. Several technical papers and standards within AMPP provide more information on this method.