In glass bead blasting, the grade you put in the pot decides almost everything downstream: how fast
the job runs, what the surface profile looks like, how much media you burn through, and whether the part
comes out cosmetic or scrap. Pressure, stand-off and angle are all adjustable at the nozzle. The grade is
not – it is a purchasing decision, and by the time the media is in the machine it is too late to change it
without stopping the job.
Glass beads do not cut the way sand or steel grit cut. A round bead peens: it hammers the
surface with a controlled, overlapping impact and produces a satin finish without removing base material
directionally. That is why the process protects dimensions, and also why the wrong grade produces either a
finish that will not take a coating, or a roughness nobody specified.
This guide covers the units, the grades, what each one does, and how to match grade to job. The
dimensional and roundness numbers referenced here come from the U.S. military performance specification for
blasting and peening glass beads, MIL-PRF-9954B, which is the reference most industrial buyers still quote
against.
1. Mesh, micron and millimetre: one table, three units
Mesh is the number of openings per linear inch in the sieve that the beads passed through. This makes it
inverse to particle size: a 30 mesh bead is coarse, a 200 mesh bead is fine. That single
fact causes more ordering errors than anything else in this industry.
Mesh numbers alone are also ambiguous, because ASTM, Tyler and various national sieve series do not
align exactly. This is why experienced buyers specify in microns, or give both. The table below is the
conversion set we work with in practice:
| Mesh | Micron (µm) | Millimetre (mm) | Typical framing |
|---|---|---|---|
| 18 | 1,000 | 1.00 | Coarsest industrial grade |
| 20 | 850 | 0.850 | Very coarse |
| 30 | 600 | 0.600 | Coarse |
| 40 | 425 | 0.425 | Coarse-medium |
| 50 | 300 | 0.300 | Medium |
| 70 | 212 | 0.212 | Medium-fine |
| 80 | 180 | 0.180 | Fine |
| 100 | 150 | 0.150 | Fine |
| 120 | 125 | 0.125 | Very fine |
For procurement against a specification, MIL-PRF-9954B Table I defines the size ranges together with a
minimum roundness for each band – roundness rises as the beads get finer. The practical consequence
is that fine grades are held to a stricter shape standard than coarse ones, because a fractured fine bead
shows up immediately on a cosmetic surface.
| MIL grade | U.S. sieve | Micron range | Minimum roundness |
|---|---|---|---|
| 3 | 20–30 | 850–600 | 65% |
| 4 | 30–40 | 600–425 | 70% |
| 5 | 40–50 | 425–300 | 70% |
| 6 | 50–70 | 300–212 | 80% |
| 7 | 60–80 | 250–180 | 80% |
| 8 | 70–100 | 212–150 | 80% |
| 9 | 80–120 | 180–125 | 80% |
| 10 | 100–170 | 150–90 | 90% |
| 11 | 120–200 | 125–75 | 90% |
| 12 | 140–230 | 106–63 | 90–95% |
| 13 | 170–325 | 90–45 | 95% |
MIL grade 8 (70–100 mesh, 212–150 µm) is the most widely used band in general
industrial work, and the one most frequently quoted on purchase orders. If you are unsure where to start, it
is usually the band to test first. A fuller treatment of sieve work and what the percentage columns on a
grading report mean is in our guide to reading a mesh-to-micron sizing report.
2. What each grade actually does to the surface
The three families behave differently enough that they are worth thinking about separately:
- Coarse (18–40 mesh / 1,000–425 µm). Fastest removal per pass and the
deepest profile. Used for heavier cleaning, removing light rust and thin mill scale, and knocking down
machining marks quickly. Also the most aggressive on thin sections and soft alloys – it is the grade most
likely to distort or dimple a delicate part. - Medium (50–80 mesh / 300–180 µm). The general-purpose band.
Deburring, blending tool marks, producing an even satin finish, and preparing a surface that will take a
coating. This is where most production blasting sits. - Fine (100–325 mesh / 150–45 µm). Cosmetic and near-cosmetic finishing.
Produces a soft, uniform matte. Slower, more sensitive to pressure, and it will not remove anything
meaningful – fine grades refine a surface, they do not clean one.
One point that surprises buyers new to the process: fine grades are not simply “better”. If a surface has
anything to remove, a fine grade will polish the contamination instead of removing it. Grade follows the
job, not the other way round.
3. Grade selection by application
The table below maps common blasting objectives to a starting grade. It is a starting point for a sample
test, not a specification – the correct answer always depends on the substrate, the condition of the
surface, and the finish called for on the drawing.
| Objective | Starting grade | Mesh | Micron |
|---|---|---|---|
| Heavy rust, thick scale, old coatings | Angular media first | — | — |
| Light rust, thin scale, general cleaning | Coarse | 20–40 | 850–425 |
| Deburring, removing machining marks | Medium | 50–80 | 300–180 |
| Satin / decorative matte finish | Medium–fine | 70–120 | 212–125 |
| Polish preparation, cosmetic finish | Fine | 100–325 | 150–45 |
The first row is not a grade recommendation, it is a warning. Glass beads cannot remove thick rust,
heavy mill scale or intact old coatings – a round bead peens over the top of them. Where a specification
calls for Sa 2.5 / SSPC-SP10 the surface has to be cut back with an angular abrasive (alumina, garnet or
steel grit) and the glass bead stage follows as a refinement. Trying to do the cutting stage with beads
burns media, time and sometimes the part.
4. Grade and the surface roughness you will actually get
Glass bead blasting typically produces a surface roughness in the region of Ra 0.8–3.2
µm, with the coarse end of that range coming from coarse grades and higher pressures, and the
fine end from fine grades at low pressure. The important practical point is that specifying an unnecessarily
tight roughness on the drawing is a common way to generate rejects: if the specification asks for a profile
finer than about 32 µin Ra, achieving it reliably becomes a process-development problem rather than a
media problem.
Roughness is not set by grade alone. Pressure, nozzle stand-off, angle and the condition of the media all
move it. Running the same grade at 2 bar and at 6 bar produces two visibly different finishes. If a finish
drifts during a shift, check the media condition and the stand-off before changing anything on the
regulator.
5. Matching grade to pressure and nozzle
Glass beads are a finishing media and work best inside a fairly narrow pressure band – broadly
3.5 to 5 bar (about 50 to 73 psi), with many shops starting a new job around 50 psi.
Above roughly 80 psi the beads stop bouncing and start fracturing.
A fractured bead does three things at once: it removes the round shape that produces the smooth finish,
it creates sharp fragments that can scratch or embed in the part, and it raises the dust load inside the
cabinet. When a job is running too slowly, the answer is usually a coarser grade rather than more pressure –
raising pressure past the band buys removal rate at the cost of the finish and the media. Our note on
what blasting pressure glass beads actually need covers this in more detail.
Nozzle discipline matters as much as grade. Stand-off is normally held at 150–300 mm
(6–12 in) and kept constant; moving closer digs and over-peens, moving further gives an uneven
pattern. The blast angle is held close to 90°. Fine grades are less tolerant of both, because the same
pressure that is gentle on a 200 µm bead will shatter a 50 µm one.
The compressed air itself is part of the grade decision. Moisture makes beads clump and the feed pulse,
and no regulator setting repairs wet media. A dew point below roughly −10 °C is the usual working
target, and media stays in sealed packaging until it goes into the pot.
6. Grade and media consumption
Coarse grades remove material faster per pass but fracture faster too, so the make-up rate is higher.
Fine grades last longer in circulation but move far less work per hour. The cost picture is therefore not
“coarse is expensive” or “fine is cheap” – it depends on how much media you have to add to hold the finish
you specified.
Two operating points drive consumption more than grade choice:
- Run the classifier. Every impact rounds a bead slightly and eventually fractures it.
The classifier continuously pulls sub-size broken material out of the working mix. Skip it and sharp
fragments stay in the charge, scratching surfaces and contaminating the finish. - Top up on condition, not on a calendar. A practical trigger is when broken and angular
material in the working charge climbs past roughly 15–20%. Past that point the charge
is a mixture of beads and shards, and the finish reflects it.
7. Why grade logic differs from angular media
With angular abrasives, coarser generally means a deeper anchor profile, and the profile is the point –
coatings need something to key into. With glass beads the logic is inverted in one important way: a round
bead does not cut a profile at all in the anchor-tooth sense. It produces a dimpled, work-hardened surface
with low stress concentration, which is why bead blasting is used where a cut profile would be harmful or
where the surface is the finished product. If you are also weighing a media switch, our comparison of
glass beads against sand sets out what actually changes.
The practical rule that follows: choose the coarsest angular media that gets the cleaning job done, then
choose the glass bead grade that produces the finish the drawing asks for. Do not ask a bead to do an
angular abrasive’s job.
One equipment constraint is worth stating plainly here. Glass beads and steel shot or grit should not
share a machine cabinet unless it has been thoroughly cleaned between media. Trace steel contaminates
stainless and aluminium and sets up delayed corrosion that shows up weeks later, long after the part has
shipped.
8. Running a sample test: a three-step selection method
Grade selection is settled faster with a small trial than with a long specification discussion.
- Narrow to two or three candidates. Pick one grade either side of your best guess – for
example 50, 80 and 120 mesh – rather than testing five grades at once. - Blast identical test pieces. Same substrate, same pressure, same stand-off, same
duration. Change one variable at a time, and record what you changed. - Judge against the actual requirement. Does the surface take the coating or the
adhesive, does it match the approved sample, and does it pass inspection for embedded fragments and
distortion? The coarsest grade that passes is usually the most economical.
Where a coating or anodising step follows, blast and coat the test pieces, because some finish problems
only appear after the next process.
9. Five grade-selection mistakes that keep showing up
- Treating a higher mesh number as “better”. A higher mesh number is a finer bead. It is
not an upgrade; it is a different tool. - Quoting mesh without microns. Sieve series do not align exactly across standards, so
mesh alone leaves room for a wrong shipment. - Using a fine grade to clean. Fine beads refine. If there is rust, scale or coating to
remove, they will polish it rather than take it off. - Raising pressure to fix a slow job. Above the working band that fractures the media and
damages the finish. Change the grade instead. - Sharing a cabinet with steel media. Trace steel contamination causes delayed corrosion
on stainless and aluminium.
Several of these overlap with the wider list of process errors that wreck a blast finish, covered in our
guide to the seven mistakes that ruin a glass bead finish.
10. Common questions
Is 18 mesh coarser than 100 mesh? Yes. Mesh is openings per inch, so 18 mesh
(1,000 µm) is coarse and 100 mesh (150 µm) is fine.
Can I convert mesh to micron on my own? Approximately, yes – see the conversion table
above. For purchase orders it is safer to specify both, or to specify a standard grade band.
Can glass beads remove rust? Light rust and thin scale, yes. Thick rust, heavy mill
scale and intact old coatings need an angular abrasive first; beads then refine the surface.
Should I use one grade or several? Many shops run two: a medium grade for general
cleaning and blending, and a fine grade for cosmetic work. That covers most production mixes.
How do I know when to change the media? Test the working charge for broken and angular
content. Somewhere past 15–20% the charge stops behaving like beads.
11. Choosing a grade for your job
If you can describe the substrate, what is on the surface now, and the finish you need, the grade usually
follows quickly. Tell us the material, the current surface condition and the required result – and if a
coating or anodising step follows, mention that too, because it changes the answer. We supply glass beads
across the full range from 18 mesh to fine finishing grades, and we are happy to talk through a starting
point for a trial.
Grade choice is only half of it – the operating discipline around it decides whether the grade performs
week after week. Our guide to
glass bead blasting precautions covers the pressure ceiling, media condition and
contamination rules that sit behind a repeatable finish.