I dedicate this article to Erdem and Doğan…
For decades I have carried out shot blasting of many yachts with garnet, sand and other projectiles. In this article, I would like to summarize some of my experiences.
Sand blasting and shot blasting is the technique of cleaning and abrading surfaces of metals, glass and glasslike materials, wood and plastics. Compared to a grinding wheel, needle gun, power brush, solvent cleaning and other methods, it requires a complex organization, but it gives very successful results unreachable by any other method.
In particular when it comes to surface preparation of metals there is no other method that will come close to it.
The Principle of Shot Blasting
Sand, garnet and various other projectiles are blasted by pressurized air at very high speed on the surface to be treated. The speed is achieved by a so called Venturi nozzle, made of particularly hard and resistant materials. During this process, the projectiles remove paint, rust and other unwanted coatings, and form a peculiarly roughened surface with micro craters.
Projectiles / Blasting Ammunition
The most widespread projectile in Turkey used to be quartz sand. In the northern regions the “Podima sand”, the sand obtained from the Black Sea beaches, was used. Salt in sand is harmful for blasting work; also, beach sand grains with comparatively rounded surfaces formed through long time natural grinding diminish the effect of blasting. On the other hand, quartz fragments of 1 to 3 mm in size – a by-product of quarries – used to be a very suitable projectile. In our region it was quarried in Çine. The stain of this sand is pure white to light violet.
However, sand crumbles away during blasting and cannot be reused. More important, sand is very harmful to health (silicosis) and has been predominantly replaced by other projectiles. These are garnet (jacinth, almandine, andradite etc.), copper slag (Kupferschlacke) or minerals with no, or little, quartz. Garnett is a by-product of gem stone production and is imported to Turkey, among others, from India.
In addition, dozens of projectiles, ranging from CO2 ice to fruit husks can be used, depending on substrate and contamination. In some applications, crushed nutshells are used. Projectile volume, weight and high consumption, its long distance procurement are economically impeding factors.
Typical compressors are screw compressors that produce air in the order of 300 – 500 m3/h.
Nozzles are made of very hard components (for example, boron carbide) with a bore of 8 to 12 mm and shaped such to create a Venturi effect. Under suitable conditions, the nozzle may propel the projectile to speeds up to 200 km/h.
This is a simple, pressure resistant vessel with not too many features; it usually has a mixing valve at the lowest point of its funnel like base. This valve tends to clog up easily and should be of a built which can be maintained easily.
Operator and the Protective Equipment
Being a blasting operator is tough; especially during summer months it requires much energy. The operator’s level of instruction/ training need not be high. However, it requires a meticulous personality.
The sand blasting operator inhales forced air; this air is in general branched out of the main compressor and is then filtered. The operator protects his/her head with a helmet made from light metal or fibreglass. Goggles are made of double glass; the sacrificial exterior glass protects the interior one and is replaced frequently during action. The operator’s body is protected by a leather apron, boots and gloves. The inhaled air needs to be conditioned to human use.
Hoses and Hose Fittings
Textile reinforced rubber hose, designed to resist about 10 bars, is used. The hose downstream of the mixing vessel is one size larger than the air hoses: This hose section is replaced frequently due to abrasion wear. The hose fittings should be of quick coupling type and with a bore equal or larger than the hose bore.
Blasted surfaces have to be coated at once. The primer in the example is zinc phosphate and leads to excellent results.
Surfaces Suitable for Blasting
Blasting of Steel
Steel surfaces are blasted to remove surface rust and form a roughened surface. With roughening, the surface “enlarges” by about 300% This is very beneficial for the coatings to bond to the substrate. Paints that bond the best to a blasted surface are epoxy based primers. Epoxy zinc phosphate is a very successful primer; it also protects anodically the metal substrate. The surface quality after blasting is classified between SA1and SA3. See table below. The best surface obtained in field conditions is the class of SA2.5. A boat hull blasted to class SA 2.5 and painted with epoxy based paint appropriately will only require for the following 6 to 8 years fixing of the odd damages. There will be no significant rusting and the paint system will not come off as long as the damages are treated in time.
Primer should be applied within 2 to 4 hours of blasting depending on the weather. Blasted metal appears light grey, homogeneous and has a silky texture. The surface is very sensitive until it is painted: The surface must not get in touch with thinners, acetone, trichloroethylene, clothes, brushes, etc. The surface must not be touched – it may be “swept” by dry, pressurized air.
With cast iron (ballast keel) sand blasting may give inconsistent results depending on the pore constitution of the surface. Rust and surface deterioration in the deep pores of some casts cannot be removed completely with any method – including sand blasting.
Blasting of Aluminium
Aluminium, too, can be blasted in preparation for coating. However, contrary to steel, disk grinding aluminium surfaces to rough grade (say 36) and using the right primers (epoxy zinc chromate or barium chromate) will give good results with aluminium substrates. However, with aluminium, too, the best results are obtained by sand blasting. In aluminium the surface oxidation is very thin and will not show. Therefore, applying a primer soon after blasting is a must.
In the United Kingdom, wooden surfaces with thick multi layer coating are rejuvenated and prepared for new coating are blasted with suitable, “soft” projectiles, as a matter of routine. This work requires a sensitive hand.
Shot blasting is used on GRP substrates, especially those damaged by osmosis. When fiberglass affected by hydrolysis (of osmosis) is subjected to blasting, the delaminated regions close to the surface are blown up selectively and the sound areas are affected only little. When compared to alternative preparation methods like the Gel Plane or disc grinding, surfaces prepared by blasting require much more smoothing labour and materials. However, the blasted and expanded surface improves curing times and is not only better for drying up of the fiberglass; it also leaves the sound surfaces untouched. This method requires very able blasting operators as, GRP may suffer unnecessary harm easily.
|In these pictures one can see preparations for shot blasting, like blinding of apertures and priming after partial sand blasting of an aluminium deck. The paint applied is zinc chromate and is phased out.|
Some points to pay attention
- Blasting requires high air flow rate. Since the rented compressor operators are wary of their machines, this flow rate may often not be obtained. As a result the blasting quality is reduced , the operation lasts longer and the projectile consumption may reach frightening amounts. The compressor should always be chosen one size bigger than theoretically required. Measuring flow rate is difficult, but one can measure the pressure at the nozzle during blasting and this figure will give some idea about the flow rate.
- Worn out nozzle increases projectile consumption.
- Some projectiles, in particular sand, crumble into very small bits, virtually dust, and this dust penetrates, through the smallest cracks and openings, into the boat, and will find its way everywhere, including electronic equipment and engine pistons. Since, when blasting steel, iron oxide (rust) mixes into this dust objects around the blasting operation will suffer if no precautions are arranged for. At best they will be stained due to the iron oxide. Claims by affected third parties are not uncommon. This is less of a problem with projectiles like garnet.
- The goggle glass of the blasting operator must be changed at short intervals. Blasting turns the goggles opaque in short time, impairing vision of the operator. Such a “blindfolded” operator forms hazard to himself, to others and to property.
- Reusing sand will reduce productiveness. Hard projectiles, like garnet and other semi-precious minerals may be reused.
- Unsieved projectiles as well as humid projectiles will clog the nozzle. Drying them up is very difficult and inefficient.
- Sea sand, unless washed, ruins in no time surfaces sensitive to humidity, like metal surfaces.
- It is better to apply two coats of primer to the blasted surface soon after operations, preferably by brush or by airless equipment. Since with one coat of primer some pores will be missed, surface deterioration will be initiated.
- If a blasted surface could not painted in time and properly, right before commencing painting one should apply a fast “sweep blasting”, and then continue with the painting.
Dr. Yusuf Civelekoğlu, CMI
Photos: marineSOLUTIONS archive
|Name||Description||Sweden Standard||English Standard||SSPC Codes||NACE||CDN. Govt.|
|White Metal Blasting||All the visible rust, slag and mill scale, paint and other contaminators are removed. The metal will appear white/grey. This is the highest cleaning degree. This method is applied when maximum paint bonding is necessary due to harsh environmental conditions. One example would be the surfaces continuously immersed in water or the liquid chemicals.||Sa. 3||BS4232|
|SSPC.SP5||NACE #1||31 GP 404|
|Near White Metal Blasting||In this method, oil, grease, dirt, mill scale, rust, corrosion products, oxides, paint and other kinds of foreign materials are removed from the surface. However, rust stains, exceptional light shades resulting from mill scale oxides or paint either weakly or strongly bonded and other coating residues are acceptable. For 25 mm x 25 mm piece of surface, 95% will be cleaned of visible residuals and the problems in the rest will remain limited to the light color spots or stripes mentioned above. From a practical perspective, this method represents the best surface quality a used metal structure will obtain.||Sa. 2 ½||BS4232|
|Commercial Blasting||In this method, all the oil, grease, dirt, mill scale and other types of foreign materials are removed from the surface. However, rust stains, exceptional light shades, light stripes and light color spots resulting from mill scale oxides or paint either weakly or strongly bonded and other coating residues are acceptable. If there is pitting on the surface, some light rust and paint residuals may remain in the bottom of the pits. For 25 mm x 25 mm piece of surface, minimum 67% must be cleaned of visible residuals and the problems in the rest should remain limited to the light residuals mentioned above.||Sa. 2||BS4232|
|SSPC.SP6||NACE #3||31 GP 404|
|Sand Brushing||In this method, all the oil, grease, dirt, crusted rust, mill scale, rust and paint residuals that have not penetrate to the surface and other types of foreign materials are removed from the surface. The mill scale and rust that did not bond to the surface strongly, paint and other surface coatings are allowed to remain. However, what is sought in this method is blasting of the mill scale residuals and rust at the level that will allow for plenty of metal homogenously spread to the surface to appear from underneath.||Sa. 1||Light blast to brush-off||SSPC.SP7||NACE #4||31 GP 404|
Nozzle and consumption relations
|Pressure (in bars and at the nozzle)|
|Nozzle size (mm)||4||5||5,5||6||7|
 This figure is very approximate; the matter will excite fractional geometry aficionados.