What is Dry Abrasive Blasting?
Dry abrasive blasting, generally referred to as sandblasting, is a process by which ageing paint can be removed to condition the under coated material (referred to as the substrate) ready for the application of new paint. This is achieved by propelling an abrasive material at high pressure on to the painted surface.
Blasting is used for a variety of different applications in everyday life - from restoring post boxes, classic cars or deburring components, to preparing the surfaces of ships and bridges before new protective coatings are applied.
The concept isn't new - the sandblasting process traces back to a machine patented in 1870 to remove rust and paint. It was decided that hand tools such as grinders and sanders just weren't quick enough to remove paint from large areas and the sandblasting process was born.
What equipment is used when Blasting?
Air is supplied from a suitable compressor and this can be either connected directly to the blast pot or into an Air Distribution Manifold (ADM). The ADM acts as a large buffer of air which can be drawn off as required by an abrasive blast machine (blast pot). These are often used if more than one blast pot is in operation to ensure air is evently distributed between pots.
Connected to the blast pots are generally three hoses; the green and yellow hoses operate a switch (deadman's handle) which is controlled by the blaster to start and stop operation, whilst the black, thicker blast hose carries the mixture of air and abrasive.
The blasting process is controlled by the operator who aims a nozzle at the areas to be blasted and stripped of coating. It is essential that the blaster wears appropriate Personal Protection Equipment (PPE) such as a blast helmet, suit, gloves and boots. The helmet is supplied with breathing air via an air filter which ensures the blaster breathes quality and safe air - further PPE is also recommended to ensure maximum protection.
The Blasting Process
In simple terms, a suitably chosen abrasive (such as steel shot or grit) is poured into the top of the blast pot and allowed to drop into the depressurised vessel which is then pressurised by the blaster. The abrasive falls through a control valve (grit valve) and into an inverted 'T' shaped mixer where the high pressure compressed air propels the abrasive through the blast hose and emits it with high speed at the working nozzle.
The blasting process requires hardened abrasive to impact the surface with high energy in order to prepare it for a subsequent coating. The preparation of the surface is to remove mill scale from bare steel or previous coatings. An essential part of this process is to control the parameters so that a suitable profile is created on the substrate. A suitable profile offers a key for the new paint to adhere to - if is too big it can become costly in paint, however if it is too small then it can prevent the paint from adhering - consequently, it is important to ensure an optimum profile to ensure long lifespan of the protective coating.
An optimum profile for a paint system is published on the paint technical data sheet which influences the abrasive chosen and the set up of the blast system.
To remove protective coatings and create a profile, a nozzle pressure of 7bar is commonly used.
Setting up the Blast System
In order to achieve this, the system needs to be balanced with careful choices made on the compressor specification, pressure and nozzle size. The ideal scenario is to have a large blast nozzle to blast the surface quicker, however there are many design considerations including the blast system's ability to carry the necessary volume of air (CFM) to achieve the desired nozzle pressure.
Elcometer have designed a series of blast pots which allow the use of larger nozzles to achieve maximum efficiency. Available with a maximum working pressure of 15bar (217psi) and engineered for ultimate productivity on large infrastruce projects, the full range can be viewed by clicking here.
With carefully designed blasting systems, the blaster can quickly and easily remove ageing coatings to enable a new protective coating to be applied, ensuring the structure is returned to service in good time.