Garnet blasting abrasive for Sa 3 surface preparation
GRM Garnet is crushed 98% almandine garnet for blasting steel structures, tanks, pipelines, ships and industrial plant. Every blasting grade achieves Sa 3, the top ISO 8501-1 cleanliness grade, and leaves very little salt behind: 10–15 ppm chlorides and 0.5% free silica.
Count the cost per square metre. Not per tonne.
Contractors can blast with sand, slag, steel, glass, mineral sands or garnet, and each has a price per tonne. What decides the cost of a job is how fast the abrasive cleans, how much of it is used, what it leaves on the steel and in the air, and what has to be disposed of afterwards.
Hard grains cut fast. Heavy grains hit hard.
At 7.5–8.0 on the Mohs scale, GRM Garnet is harder than slag, sand and glass, and above the 6.5–7.5 range commonly published for garnet abrasives.10,11 Hardness decides how fast a grain cuts and how long it keeps its edge.
Weight decides how hard each grain strikes. At the same nozzle speed a denser grain carries more energy into the surface. With a specific gravity of 4.0, a GRM grain is about 50% heavier than a grain of silica sand of the same size, and 60% heavier than crushed glass.10
Purity sets how much of each tonne works
GRM Garnet is 98% almandine. The other 2% is a little muscovite, quartz and ilmenite. With very little soft or brittle material to shatter on impact, less of what you buy turns to dust and more of it cleans steel.
The cheapest tonne can be the dearest job
In the NIOSH abrasive programme, silica sand had the lowest price per ton but the highest cleaning cost per square foot.4 An abrasive that cleans slowly and is used heavily costs more once labour, equipment time, clean-up and disposal are counted.
Published values for each abrasive type.10,11 Bars show the published range. Mineral sands are staurolite and olivine, published as one group.
Don’t take our word for it.
NIOSH commissioned a head-to-head study of silica sand and its substitutes, first in the laboratory and then on real structures. Garnet cleaned faster than every other abrasive in the field.
Source: KTA-Tator Inc. for NIOSH (1999), Evaluation of substitute materials for silica sand in abrasive blasting. Averages for each abrasive type, converted from ft²/h and lb/ft². Results describe garnet as a type, not GRM Garnet specifically.
Sa 3 on every blasting grade. From 8/12 to 120#.
ISO 8501-1 grades blast-cleaned steel on a four-step scale. The top step, Sa 3, is blast cleaning to visually clean steel: no visible oil, grease, dirt, mill scale, rust, paint or foreign matter, and a uniform metallic colour.2 Every GRM blasting grade achieves it with correct blasting parameters.
Light
Loose mill scale, rust and paint removed.
Thorough
Most contamination removed; any residue is firmly adhering.
Very thorough
Only slight stains in spots or stripes remain.
Visually clean steel
Uniform metallic colour. Every GRM blasting grade gets there.
Sharp, sub-angular grains
GRM Garnet is mined from hard rock and crushed to size, so its grains carry fresh fracture faces and edges. A sub-angular grain strikes with an edge and cuts a crisp anchor profile for the primer to key into. Its toughness helps it keep that shape through the blast instead of breaking down to dust.
Less embedment, a cleaner surface
Friable abrasives shatter on impact and can lodge fragments in the steel, which later push up through the new coating. Slags can also leave a dark residue that compressed air will not shift.12 Tough garnet grains tend to rebound intact.
Low salt protects the coating
Coatings fail from underneath. Soluble salts left on blasted steel, chlorides above all, draw water through the paint film and drive osmotic blistering and under-film corrosion.12 ISO 11126-10, the specification for almandite garnet blast abrasives, caps water-soluble chlorides at 25 ppm and the conductivity of a water extract at 250 µS/cm.1
GRM Garnet typically measures 10–15 ppm and 100–150 µS/cm, 40–60% inside both limits. Most coating specifications also limit the salt left on the steel itself, measured with ISO 8502-6 and ISO 8502-9; a low-chloride abrasive makes that limit easier to meet.12
Eight grades, coarse to fine.
Coarse grades cut a deeper profile and suit heavy coatings and heavy rust. Finer grades suit thinner coatings and new steel, and give a tighter, more even profile. Every grade has a published sieve analysis, so the profile and the feed through the pot stay consistent.
| Grade | Typical size, d10–d90 | Best for |
|---|---|---|
| 8/12 | 1.73–2.24 mm | Heavy coatings, heavy rust and mill scale; deep anchor profiles |
| 8/80 Alpha Blast | 0.25–2.00 mm | Alpha Blast. One broad grading: coarse grains cut depth while fine grains clean the valleys |
| 36# | 0.38–1.34 mm | Coarse-to-medium work on heavy coatings; deeper profiles |
| 14/40 | 0.65–1.54 mm | Medium-coarse blasting; also the standard filter transition grade |
| 20/60 | 0.23–0.78 mm | General surface preparation; a tighter, more even profile |
| 50# | 0.26–0.45 mm | Thinner coatings, new steel and light rust |
| 80# | 0.16–0.32 mm | Fine finishing and light cleaning |
| 120# | 0.11–0.21 mm | The finest grade, for very fine finishing and detailed work |
Confirm the profile on a test patch. The profile depth and cleaning rate depend on pressure, nozzle size, stand-off distance, the steel and its condition, as well as the grade. Follow the profile your coating specification calls for.
Less silica, no slag metals. Less dust.
Every blast job makes dust. What that dust contains depends on the abrasive, and that is the biggest health difference between them.
OSHA’s limit for respirable crystalline silica over 8 hours.7 Breathing crystalline silica dust causes silicosis, an incurable lung disease, and is linked to lung cancer.5,7 Great Britain banned sand for blast cleaning in 1950, and NIOSH has recommended against abrasives with more than 1% crystalline silica since 1974.5
OSHA’s 2017 limit for beryllium over 8 hours.8 Slags are by-products of power stations and smelters and can carry trace arsenic, beryllium and cadmium.6 OSHA identified blasting with slag containing trace beryllium as a main route of worker exposure.8
Free silica, half NIOSH’s 1% line. Garnet is a natural silicate mineral, not an industrial by-product, so it brings no slag metal load. Radioactivity is not detectable, and garnet is listed among the low-dust abrasives in the substitutes bulletin produced with NIOSH.9
Good practice still applies. No abrasive is dust-free. Engineering controls such as enclosures, ventilation and wet methods, and respiratory protection, are needed whatever the abrasive.6 Choosing garnet lowers the hazard; it does not remove the need for controls.
Recover it, reuse it. Dispose of less.
Typical reuse
Garnet can usually be recovered and reused about six to seven times, against three to four for staurolite.9 Actual reuse depends on the job, the equipment and how well spent abrasive is cleaned.
Less to bag and dispose of
Garnet used 30% less abrasive by weight per square metre than silica sand in the NIOSH laboratory trials, before any reuse is counted.3
Better visibility
Less dust in the air lets the blaster see the surface being cleaned, and less of it settles on finished work, equipment and neighbouring sites.
Spent abrasive takes on the hazards of the coating it removed, such as lead paint, so disposal rules depend on the job as well as the abrasive.
Ask every supplier for these numbers. In writing.
Here is what to request, why it matters, the benchmark to hold it against, and what GRM Garnet typically delivers.
| Ask for | Why it matters | Benchmark | GRM Garnet |
|---|---|---|---|
| Almandine content | Purity sets hardness and weight, and how much of each tonne actually cuts | As high as possible | 98% |
| Hardness | Harder grains cut faster and keep their edge for longer | ISO 11126-10 minimum: 6 Mohs | 7.5–8.0 Mohs |
| Specific gravity | Heavier grains strike the surface with more energy | Garnet commonly 3.5–4.3 | 4.00 |
| Water-soluble chlorides | Salt left on steel drives blistering and under-film corrosion | ISO 11126-10 maximum: 25 ppm | 10–15 ppm |
| Conductivity | A quick measure of total soluble salts | ISO 11126-10 maximum: 250 µS/cm | 100–150 µS/cm |
| Free silica | Breathing crystalline silica dust causes silicosis | NIOSH: avoid abrasives above 1% | 0.5% |
| Radioactivity | Some mineral sands carry radioactive monazite | Should not be detectable | Not detectable |
| Moisture | Damp abrasive bridges in the pot and feeds unevenly | ISO 11126-10 maximum: 0.2% | Non-hygroscopic |
| Grain shape | Angular grains cut; rounded grains cut less aggressively | Sub-angular or angular | Sub-angular |
| Blast finish | The cleanliness grade your coating specification calls for | ISO 8501-1 top grade: Sa 3 | Sa 3, every grade |
| Sieve analysis | Consistent sizing gives a consistent profile and feed rate | Published for each grade | 8 grades, published |
Garnet blasting: common questions.
Is garnet better than silica sand for blasting?
Garnet is harder and heavier than silica sand: GRM Garnet is 7.5–8.0 Mohs with a specific gravity of 4.0, against about 6–7 Mohs and 2.65 for sand. In the NIOSH field trials, garnet cleaned about 36% faster than silica sand, and in the laboratory trials it used about 30% less abrasive by weight per square metre.
Sand is also crystalline silica, whose dust causes silicosis. GRM Garnet has 0.5% free silica, half the 1% line NIOSH recommends.
Can garnet blasting abrasive be reused, and how many times?
Yes. With a recovery system that cleans and screens the spent abrasive, garnet can typically be reused about six to seven times, against three to four for staurolite. Actual reuse depends on the job, the equipment and how well the abrasive is cleaned between passes.
Which garnet grit size should I use on steel?
Use a coarse grade such as 8/12 or 36# for heavy coatings, heavy rust and deep anchor profiles. 8/80 Alpha Blast is a broad grading that cuts depth and cleans the valleys in one pass. 20/60 suits general surface preparation, and 50#, 80# and 120# suit thin coatings, new steel and fine finishing.
The profile also depends on pressure, nozzle, stand-off distance and the steel, so confirm it on a test patch against your coating specification.
How much garnet do I need per square metre?
It depends on the surface condition, the coating being removed, the nozzle and the pressure. In the NIOSH laboratory trials, garnet averaged about 46 kg per square metre under the test conditions, about 30% less than silica sand. Plan a large job from a trial area on the actual steel.
Is garnet safe to blast with? How much free silica does it contain?
GRM Garnet contains 0.5% free silica, below the 1% line NIOSH recommends, carries no slag metals such as beryllium or arsenic, and has no detectable radioactivity.
No abrasive is dust-free, though. Use engineering controls such as enclosures, ventilation and wet methods, and respiratory protection, whatever the abrasive.
Can garnet be used for wet blasting and in blast cabinets?
Garnet is commonly used in wet and dustless blasting systems and in blast cabinets. GRM Garnet does not absorb moisture, so it feeds evenly. Finer grades usually suit cabinets; ask us which grade fits your equipment.
Does spent garnet need special disposal?
Garnet itself is a natural mineral, but spent abrasive takes on the hazards of the coating it removed, such as lead paint. Disposal rules depend on the job as well as the abrasive. Using less abrasive per square metre and reusing it reduces the tonnes to dispose of.
Specifications and further reading.
Other uses of GRM Garnet
Sources
- ISO 11126-10:2025. Preparation of steel substrates: specifications for non-metallic blast-cleaning abrasives, Part 10: Almandite garnet.
- ISO 8501-1. Preparation of steel substrates: visual assessment of surface cleanliness, rust grades and preparation grades.
- KTA-Tator Inc. for NIOSH (1999). Evaluation of substitute materials for silica sand in abrasive blasting, Phase III.
- Atkins and Greskevich, NIOSH (1998). Evaluation of substitutes for silica sand in abrasive blasting.
- NIOSH Alert 92-102. Preventing silicosis and deaths from sandblasting.
- OSHA FS-3697. Protecting workers from the hazards of abrasive blasting materials.
- OSHA FS-3682 (2018). Respirable crystalline silica standard.
- OSHA (2017). Occupational exposure to beryllium, final rule.
- New Jersey Department of Health with NIOSH; Michigan State University (2006). Silica substitutes for abrasive blasting.
- Graco. How to choose the right abrasive.
- Reade Advanced Materials. Mohs hardness of abrasives.
- Institute of Corrosion, Aberdeen (2026). Surface preparation and abrasive selection.
GRM Garnet values are typical and do not constitute a specification. Third-party figures describe garnet or other materials in general, not GRM Garnet, unless stated.