Blasting grit for every industrial application

Which blasting grit gives the best result on your workpiece, in your installation? Straaltechniek International supplies all types of blasting grit and abrasive blasting media, advises on the right combination of grit, grain size and blasting process, and tests this in advance in our own blasting lab.

What is blasting grit?

Blasting grit is an angular, sharp-edged abrasive that is propelled against a surface by compressed air or a blast wheel in order to clean, derust, strip paint or roughen it. Its sharp edges cut into the surface and create a roughness profile to which paint, powder coating or thermal spray metallising can adhere. In everyday language this is still often called sandblasting, but real sand is no longer used: in the Netherlands it has been banned since the 1957 Sandblasting Decree.

The opposite of grit is shot: round particles that do not cut the surface but compact it. You use shot for a smooth finish or for shot peening. This page covers both, because the choice between grit and shot is the first step in every blasting process.

Which blasting grit for which application?

The choice depends on three things: the material of the workpiece, the result you want to achieve and the installation in which the grit is used. The table shows the most common combinations. Not sure? We test it in advance on your own workpieces.

Blasting grit as surface preparation for coating: rust grade and cleanliness grade

If the workpiece is coated, painted, powder coated or metallised after blasting, the surface preparation determines how long the coating will last. Two concepts from the ISO 8501-1 standard are key here: the rust grade of the steel as it comes in, and the cleanliness grade (Sa) you want to achieve after blasting. The coating supplier usually specifies the required cleanliness grade.

Sa 2½ is the standard requirement for most industrial paint systems. The heavier the rust grade (C or D), the harder and more angular the grit needs to be to clean out the pits as well. Mineral grit such as corundum gives a clean grey, sharp profile and almost always exceeds Sa 2½; blasted work can also be left standing longer afterwards without losing cleanliness. Chilled iron grit, by contrast, attracts moisture: work blasted with it must be processed further quickly.

Grain size and roughness: the right anchor profile

During blasting, every particle strikes a small dent in the surface and pushes the surrounding material upwards. This creates a pattern of peaks and valleys: the anchor profile. The coating must cover the peaks and fill the valleys. If the peaks protrude through the coating, rust will soon start there. If the profile is too shallow, the coating adheres poorly and will detach.

Three properties of the blasting grit determine the profile:

• Grain size: a coarser grain creates a deeper profile and works faster; a finer grain gives a more even, smoother surface.
• Shape: angular grit creates sharp peaks and valleys (a true anchor profile); round shot creates shallow dimples, like those on a golf ball.
• Bulk density and hardness: a heavy, hard grain (steel grit, 4.5 kg/l) strikes deeper than a light one (corundum, 2 kg/l) of the same grain size.

The rule of thumb: choose the finest grain that still achieves the specified profile. Coarser than necessary costs more abrasive, more paint to fill the deeper valleys and more blasting time. The specified profile is usually stated in the technical data sheet of the coating, expressed in µm or as fine, medium or coarse according to ISO 8503. Grain sizes are designated according to SAE J444 (steel shot and grit, for example G25 or S330) or FEPA (corundum, for example F36). You can read more about measuring roughness in our article on roughness and the right abrasive.

Types of blasting grit

Grit is angular. Below are the types we supply, from metallic to mineral.

Round and soft blasting media (not grit)

These media are round or soft and do not cut. You choose them for a smooth finish, for shot peening or for delicate workpieces. Shot peening involves stricter requirements for grain size, uniformity and hardness; read more about shot peening media.

Comparing blasting grit: shape, hardness and reuse

Values based on product data sheets and technical literature; the exact number of cycles depends on working pressure, settings and cleaning. Metallic grit (4.5 kg/l) strikes harder and removes thick layers faster than mineral grit (2 kg/l), but mineral grit contains many more particles per kilo and reaches right into the pores.

Choose the finest grain that still achieves the required profile: coarser than necessary costs abrasive, paint and time. How grain size and roughness are related is explained in roughness and the right abrasive.

Single-use abrasives: slag, calcium silicate and crushed glass

Besides reusable grit, there are single-use abrasives: slag, calcium silicate (often sold as ‘blasting sand’ or ‘blasting grit’ in DIY stores) and crushed glass. They are cheap per kilo and suitable for open-air blasting, where the abrasive is swept up and disposed of after use.

In a blast room or blasting hall they are rarely the best choice. They break down into dust after one or a few cycles, consumption per m² is high and disposal also costs money. Moreover, slag is becoming scarcer. If you blast regularly, reusable steel grit or corundum in an installation with abrasive cleaning is almost always cheaper per blasted m², and produces less dust in the blast room.

Reusing blasting grit: calculate per m², not per kilo

Metallic grit lasts hundreds to thousands of cycles, mineral grit dozens, provided it is cleaned after every cycle. Dust, paint residues and broken particles must be removed, otherwise the blasting rate drops and dust generation increases.

The price per kilo therefore says little. What counts is how much grit you use per blasted square metre. As an indication for recyclable grit in a blast room:

Garnet is cheap per kilo, but you use six times as much of it per m² as steel grit. With mineral grit, consumption also doubles between 4 and 6 bar working pressure. In practice, the blasting pressure and abrasive metering valve are often set too high; we measure this too when we carry out trial blasting of your workpieces.

The grit itself is only part of the cost per m². Blasting speed, machine wear and the disposal of used grit count as well. Stainless steel grit, for example, is expensive per kilo but can work out favourably overall: it lasts a long time and machine parts wear less.

We build the installations that make reuse possible: the abrasive separator in the blast room, the abrasive recovery system for large blasting halls and the vacuum recovery system for on-site blasting. We also take care of the disposal of used blasting grit.

Working pressure, air consumption and blast nozzle

The same blasting grit can work far more economically in one installation than in another. Three settings make the difference:

• Working pressure: with mineral grit, consumption roughly doubles between 4 and 6 bar. A higher pressure is therefore not automatically faster or cheaper.
• Abrasive metering valve: if it is opened too far, more grit passes through the nozzle than necessary, without improving the result.
• Blast nozzle: the bore determines how much air and abrasive pass through it, and therefore how large the compressor needs to be. A worn nozzle becomes wider and consumes more air and grit without anyone noticing.

In practice we often find that the blasting pressure and abrasive metering valve are set too high. The roughness then becomes greater than necessary, resulting in higher consumption of both grit and paint. We supply the complete compressed air chain: BlastWizard® blast pots, compressors and compressed air treatment, blast nozzles and hoses, and we fine-tune the installation together with you.

Dust, extraction and cleaning of blasting grit

Dust is created when particles break. Hard, tough media such as steel grit, stainless steel shot and ceramic beads break down very little; mineral grit breaks down faster and produces more dust. Particles smaller than approximately 0.2 mm no longer contribute to blasting and only cause problems: poor visibility, a dirtier workpiece and a lower blasting rate.

That is why reusable blasting grit always requires good extraction and cleaning. The abrasive separator separates dust, paint residues and undersized particles from the usable grit, and the dust collector captures the dust and keeps the air in the blast room clear. With less dust, the filter cartridges also last longer. Blasting with quartz sand is strictly prohibited: the fine dust penetrates deep into the lungs and causes silicosis (a serious, irreversible lung disease).

Testing in our own blasting lab

We have a fully equipped blasting test room with a laboratory. Based on trials with your own workpieces, we advise which type of blasting grit, grain size and blasting process will give the best result. We measure the roughness, assess the quality of the grit and the machine settings, and compare the result with the required cleanliness grade according to ISO 8501-1 (for example Sa 2½). You receive a laboratory report with concrete recommendations, including the expected consumption per m².

Frequently asked questions about blasting grit