Factors to consider when choosing your abrasive.
Proper abrasive selection allows you to maximize profitability by finishing jobs faster while consuming only the minimum amount of abrasive required.
A coating is only as good as the preparation of the underlying surface.
Professional wet abrasive blasters know that choosing the right abrasive is key to achieving a coating application that lasts.
These are the factors to consider when choosing your abrasive.
As a general rule, always use the finest abrasive required to attain the required surface preparation characteristics. A fine abrasive will give you more impacts per volume. The more particles in the stream, the more work is accomplished in the same time.
When blasting concrete or wood, you don’t need a hard, expensive abrasive, or a coarse particle: crushed glass makes an excellent, inexpensive choice for work on relatively soft surfaces.
However, when preparing iron and steel for a protective coating system, there are additional considerations. Coatings adhere poorly to hard, flat surfaces, so the blaster is required to develop a pattern of indentations on the substrate to which coatings may attach, this is known as the anchor pattern.
NOTE: Before choosing your abrasive, check the coating manufacturer's product data sheet(s) to ascertain the surface conditions required for application of the coating(s), e.g. 'Iron & Steel, SSPC-SP6, 2 mil profile.'
As abrasive media strikes a substrate, the force of impact deforms the surface creating a pattern of peaks and valleys. The distance between the top of the peaks to the bottom of the valleys is referred to as the anchor profile and is typically specified in mils – 1/1000 of an inch – or microns – 1/1000 of a millimeter (1 mil = 25.4 microns).
Factors impacting the development of an anchor pattern include:
The effectiveness by which a coating bonds to a prepared surface is determined largely by the anchor profile, which provides coating systems with the sites necessary to securely anchor themselves to the substrate surface.
Profile requirements are coating-specific and should be published by the coating manufacturer. Failure to establish each coating’s profile requirements and/or validate the final profile depth analytically prior to coating application risks premature (and potentially catastrophic/costly) failure of the coating.
In the total absence of any manufacturer technical data/requirements for a given coating, consider the following rule of thumb for estimating the appropriate anchor profile depth: proper anchor profile depth is roughly equivalent to 25-30% of the TOTAL dry film thickness of the applied coating SYSTEM, but NEVER GREATER than the primer coat dry film thickness UNLESS additional coats are to be immediately applied.
An anchor profile of 2-3 mils (rarely exceeding 5 mils) is typical for industrial coatings applied over steel.
Coatings should completely fill the valleys and cover the peaks of the anchor profile for optimal adhesion.
Typically speaking, deeper profiles equate to greater anchoring of the coating system. However, excessive profiles may result in premature coating failure as well. The peaks of excessive profiles may extend/protrude through coating surfaces resulting in pinpoint rust and ultimately coating failure.
In such instances, contractors may be required to reduce the anchor profile to the proper depth by reworking the area with a finer abrasive and subsequently reapply the coating system – a tedious and costly mistake!
When choosing an abrasive, it is important to consider how the unique properties of each media will affect the surface profile and finish of the final substrate.
Once the required substrate conditions, including cleanliness level and anchor profile depth, have been established, abrasive selection may proceed.
There are five abrasive properties that impact substrate conditioning:
These properties are discussed in greater detail throughout the following sections, below.
Abrasives are commonly classified by the mesh size (e.g. grit size, microns, etc.) of their particles. The mesh size number indicates the number of mesh lines per square inch in a sieve, ranging from 6 (coarse) to 327 (powder).
Mesh sizes can be specified as a singular value (e.g. 80, 120, etc.) or as a range, for example: 30/60. In such an instance, a range of 30/60 indicates that 95% of the particles in the abrasive mix will pass through a 30 mesh screen, but not a 60 mesh screen.
Generally speaking, larger mesh abrasives are best for tougher applications like hard substrates (e.g. steel) or thick coatings, while finer meshed abrasives excel when working with softer substrates like fiberglass, concrete and wood.
All else being equal, coarser abrasives (larger particles) create more aggressive (deeper) anchor profiles. However, this benefit comes with the following trade-offs:
The most efficient approach is to use the smallest particle necessary to achieve the desired profile. Small particles clean faster, provide better coverage, and result in a more uniform profile.
Additionally, cleaning efficiency/speed may be increased through the selection of abrasives of higher density.
The most efficient approach to selecting abrasive media is to use the finest abrasive (smallest particle size) necessary to achieve the required anchor profile for the coating system to be applied.
Small particles clean faster, provide better coverage, and result in a more uniform profile. Additionally, cleaning efficiency/speed may be increased through the selection of abrasives of higher density.
A general rule of thumb for coating removal: use an abrasive with a grain/particle size equivalent to the thickness of the coating to be removed.
For example, you are tasked with removing a 20 mil coating:
(Alternative to using the chart, various online tools such as the Grit & Particle Size Converter & Particle Size Conversion Calculator may be used for direct conversion between sizes.)
Abrasive particle shape, specifically the sharpness of its face angles, will influence its ability to remove coatings or contaminants and determine how deeply it will profile substrate surfaces — particles with sharper face angles will cut more deeply than round particles of similar size.
Particles with an angular shape produce anchor patterns on the substrate surface and are effective at removing soft, pliable coatings and rust. In contrast, rounder particles produce a more uniform, peened surface, and are better suited for removing hard, brittle coatings and mill scale.
Abrasives are classified into four categories according to the shape (angularity) of their particles:
Abrasive media hardness is assessed using Mohs Hardness Scale, which ranges from 0 to 10 with larger numbers corresponding to greater hardness.
Generally speaking, harder abrasives create deeper/rougher anchor profiles, remove materials more quickly, and are more resistant to shattering as they absorb less energy upon impact, making them suitable for recovery and reuse.
Softer abrasives, like organic materials and plastics, are good for removing dirt, oil, grease and paint without creating an anchor profile in the underlying substrate but require longer to do so.
| ABRASIVE | HARDNESS (MOHS) |
|---|---|
| Plastic | 3.0-4.0 |
| Walnut | 3.0-4.0 |
| Glass Bead | 5.5-6.0 |
| Silica Sand | 5.0-6.0 |
| Crushed Glass | 5.5-7.0 |
| Coal Slag | 6.0-7.5 |
| Olivine | 6.5-7.5 |
| Staurolite | 7.0-7.5 |
| Garnet | 7.0-7.5 |
| ABRASIVE | SPECIFIC GRAVITY (g/mL) |
|---|---|
| Water | 1.00 |
| Walnut | 1.20-1.35 |
| Plastic | 1.50 |
| Glass Bead | 2.50 |
| Crushed Glass | 2.50 |
| Silica Sand | 2.65 |
| Coal Slag | 2.70 |
| Olivine | 3.30 |
| Staurolite | 3.60 |
| Garnet | 3.50-4.30 |
Abrasive media particles of higher density impact substrate surfaces with greater kinetic energy concentrated over a smaller area, yielding deeper anchor profiles than less dense media of equivalent size. They are generally capable of cleaning faster and more thoroughly with a lower application rate than low-density particles. Higher density abrasives also absorb less energy upon impact, resulting in less deformation and the potential to last longer.
Lower density abrasives will be more “gentle” on substrate surfaces than higher density abrasives of the same mesh size applied at the same blast pressure. The harder and heavier the abrasive is, the rougher the profile will be.
The final important factor to consider is abrasive media velocity as it impacts the substrate surface. By increasing the velocity, the abrasive will be more capable of imparting deeper profiles onto the surface.
Velocity is the only factor that can be easily adjusted in the field – once you have selected an abrasive with the appropriate physical properties, you can adjust your blast pressure to fine-tune the depth of the anchor profile.
The following data are approximations of general product categories and intended for orientation purposes only. Specific products may vary significantly. Check the manufacturer’s data sheets for the most current and accurate information.
Depth profile estimates are based on mild steel at 90-100 psi, nozzle at 18” at a 110 degree angle using a Terra-Tech TT15P wet abrasive blaster.
Silica sand is the original blasting abrasive, but is no longer recommended for blasting applications due to the occupational hazard silicosis. It is included here for comparative purposes only.
Garnet is a type of gemstone used in abrasive blasting (both dry & wet) due to its natural abrasive properties. Like crushed glass, garnet creates an angular profile on substrate surfaces.
Garnet is 2-4 times harder and up to double the bulk density than other abrasives such as coal slag, crushed glass, and staurolite, rendering it a very aggressive, faster-cutting alternative of greater durability that can be efficiently recycled repeatedly. Garnet blasts through coatings and surface contaminants quickly with low consumption and minimal dust generation.
With silica levels <1%, garnet is considered non-toxic, environmentally-friendly, and safe for both indoor and outdoor use. Furthermore, garnet leaves little-to-no embedded material on substrates and permits precise feathering control.
Garnet is ideal for use on ferrous and non-ferrous materials as other abrasives types may present cross-contamination concerns on aluminum and stainless steel.
Garnet comes in a range of sizes, all of which are suitable for the removal of old paint, coatings, rust, and mill scale from hard metals. Coarser sizes are excellent for use on aluminum, concrete, asphalt, and steel substrates, while finer sizes are safe for more delicate applications like brick, fiberglass, and boat paint.
| MESH SIZE | MAX PROFILE (mil) | RECOMMENDED USAGE |
|---|---|---|
| 30/60 | 2.4-4.7 | Alum., Asphalt, Concrete, Steel |
| 80 | 1.5-3.6 | Alum., Asphalt, Concrete, Steel |
| 120 | 0.5-2.0 | Alum., Asphalt, Concrete, Steel, Brick, Fiberglass Boats |
| 150 | 0.5-1.5 | Alum., Asphalt, Concrete, Steel, Brick, Fiberglass Boats |
Copper slag is a by-product of copper extraction during the smelting process. During the smelting process, molten slag is tapped off into fresh running water which instantly vitrifies the slag producing a black glass matrix with angular and shiny characteristics.
Copper slag is an aggressive abrasive very similar to coal slag but with greater density. High bulk density, combined with its angular shape, lend copper slag abrasives well to fast cutting and increased production time. It is used mostly in new steel manufacturing and maintenance blasting.
Crushed glass blasting media is a fast cleaning, aggressive, low cost, single use abrasive that is made from 100% recycled material, leading to a direct reduction in landfill waste and the use of more toxic slags.
Crushed glass is non-reactive, inert, and able to be used near and around water. It contains < 1% free silica and is free from toxic metals associated with other abrasives such as slags.
The angular shape cleans surfaces and provides a uniform mil profile with low particle embedment and delivers a cleaner, whiter metal finish than mineral sands & slags. The dust produced when using glass abrasives is translucent which allows increased visibility and operator production.
Crushed glass abrasive is excellent for removing a wide variety of coatings while creating a sharp anchor profile, and is the abrasive of choice for concrete preparation, general cleaning, restoration work, rust and scale removal. Additionally, it is ideal for blasting steel, aluminum, fiberglass, and boat paint in preparation for powder-or-standard coating application.
| MESH SIZE | MAX PROFILE (mil) | RECOMMENDED USAGE |
|---|---|---|
| 20/30 | 2.5-4.5 | Alum., Asphalt, Concrete, Steel, Brick, Fiberglass Boats |
| 30/70 | 2.0-3.5 | Alum., Asphalt, Concrete, Steel, Brick, Fiberglass Boats |
| 50/100 | 0.5-2.5 | Alum., Asphalt, Concrete, Steel, Brick, Fiberglass Boats |
Mineral sands, like olivine and staurolite, are fast-cutting, low-dusting abrasives containing < 5% silica by volume. Shape ranges from angular to sub-angular and are more cost-effective than garnet.
Mineral sands are good general-purpose abrasives used in steel fabrication and bridge maintenance to remove paint, rust, mill scale, and weathered coatings.
Coal slag is a relatively cheap, fast-cutting angular abrasive generated as a by-product of coal-burning power plants that is reclaimed and screened (to remove dust), leaving a crystalline grain. It is the most common slag abrasive used.
Coal slag contains low free-silica, has a low tendency for dust generation, and is non-metallic and chemically inert, making it a good alternative to silica and river sand. Additionally, as an industrial process byproduct, coal slag is considered a “green” abrasive because it would otherwise be discarded as waste.
Typical applications include rust, paint, weathered coatings and scale removal from steel and concrete. The variety of sizes available lend well to blasting through tough coatings and old paint layers, to removing light rust and mill scale.
Coal slag is not widely used in wet abrasive blasting because it’s considered to be a “dirty” abrasive given the high amount of “fines” (fine particles) it contains which tend to mud up in wet abrasive blasting equipment. However, its low moisture content makes it ideal for use in standard blasting equipment.
Glass beads, or glass bead abrasives, are most often used in general maintenance and blast cleaning processes. Blasting with glass beads allows the user to clean work pieces without significant surface removal. Due to its spherical shape, glass beads are also used in some peening applications. The shape also lends to a slow break down allowing for multiple turns and impacts.
The shape of glass beads allows for a four-step process in one application because they can clean, finish, peen, and in some cases even deburr. That combination provides a controlled and unique surface finish when compared to other common blasting abrasives. It leaves a nice, bright, satin finish on metallic surfaces.
Glass bead media is available in an assortment of sizes which allows for a variety of surface finishing and cosmetic textures for applications ranging from blasting, cleaning, peening, and deburring, to metal finishing and military-spec peening (AMS2431/6C).
Some specific applications for glass bead abrasives are sensitive metal surfaces, removing automotive paint, brightening grout, removing fungus and calcium deposits from tile, and polishing of various items (e.g. cast iron, stainless steel, aluminum, propellers and turbine blades).
Glass beads are chemically inert and will not leave contamination, or residue, on surfaces. However, they are not robust and will disintegrate (literally into dust) from striking substrate surfaces making this abrasive unsuitable for recovery and recycle.
Plastic abrasive media is a lightweight, angular-shaped abrasive media that can be used in a variety of applications. Amongst the softest abrasives available, plastic media is considered a faster alternative to hand-stripping and a less-hazardous alternative to chemical stripping.
Plastic media is created from either recycled plastic products or resin and comes in a variety of types, the most common being polyester (Type I), urea (Type II), melamine (Type III), or acrylic (Type V), clear-cut (Type VI) & CHP (Type VII). Each type has a different hardness to meet the application need.
Plastic media is most commonly used in the removal of paint or other coatings on several types of surfaces (aircraft, automotive, electronics, and composites) without harm to delicate substrates such as aluminum, composites, fiberglass, plastics and wood.
Plastic abrasive media has high reusability rates compared to glass and mineral abrasives.
Walnut shell blasting media is an angular, bio-based, and biodegradable abrasive used to remove paint and carbon from several types of surfaces (aerospace, automotive, bridges and buildings).
Walnut shells are soft enough to strip surfaces of grease, carbons, electrical components, and dirt without etching like harder abrasives as they don’t create an anchor profile or change the underlying substrate.
Walnut shell is a good consideration when blasting components used in food preparation equipment cleaning. Other typical applications include aircraft engines, auto body panel cleaning dies and molds, electric motors, polishing watches and jewelry, and restoring antique surfaces.
Organic abrasives such as walnut shells tend to have lower disposal costs compared to their glass or metallic alternatives.
Proper abrasive selection is key to efficient, profitable blasting operations.
| Abrasive | Mesh Sizes | Hardness | Density (SG) | Shape | Cost |
|---|---|---|---|---|---|
| Silica Sand | 6-270 | 5.0-6.0 | 2.65 | Rounded | $ |
| Garnet | 30-120 | 7.0-7.5 | 6.50-4.30 | Subangular | $$$ |
| Copper Slag | 12-80 | 7.0 | 2.80-3.60 | Angular | $ |
| Nickel Slag | 12-300 | 7.0 | 2.80-3.80 | Angular | $ |
| Crushed Glass | 30-400 | 5.5-7.0 | 2.50 | Angular | $ |
| Mineral Sands | 20-120 | 6.5-7.5 | 3.30-3.60 | Ang-Round | $$ |
| Coal Slag | 12-80 | 6.0-7.5 | 2.70 | Angular | $ |
| Glass Bead | 30-325 | 5.5-6.0 | 2.50 | Rounded | $$$ |
| Plastic | 12-80 | 3.0-4.0 | 1.50 | Angular | $$$ |
| Walnut | 6-100 | 3.0-4.0 | 1.20-1.35 | Subangular | $$$ |
Surface preparation is not an exact science and there are a multitude of variables which impact blasting performance and outcomes.
As such, it is recommended to validate the required cleaning efficacy, rate and anchor profile depth can be achieved with the selected abrasive, nozzle and blast pressure settings prior to initiating a job.
Testing can be performed on a scrap material sample or small, concealed area on the substrate and any required adjustments can be made subsequently to ensure compliance with all project requirements.
The benefit to this approach is that, once the system settings have been validated, so long as they are maintained consistent performance from the system may be expected.
| PROFILE DEPTH (mils) vs. ABRASIVE MESH SIZE | |||||||||
|---|---|---|---|---|---|---|---|---|---|
| 0.5 | 1.0 | 1.5 | 2.0 | 2.5 | 3.0 | 3.5 | 4.0 | 4.5+ | |
| Crushed Glass | 40/70 | ||||||||
| 20/40 | |||||||||
| 10/40 | |||||||||
| Garnet | 100 | ||||||||
| 80 | |||||||||
| 30/60 | |||||||||
| 20/40 | |||||||||
| Coal Slag | 30/60 | ||||||||
| 20/40 | |||||||||
| 12/40 | |||||||||
| Copper/Nickel Slag | 30/60 | ||||||||
| 20/50 | |||||||||
| 12/30 | |||||||||
