In short: blackening, grey staining, yellow spots and later rust blooms on aluminium and stainless parts are almost never an equipment fault. They come from iron contamination — free iron embedded in the surface by cast steel shot, cut wire shot, 430 shot, or by carbon-steel wear parts in the machine. The fix is two-sided: 304 or 316 stainless media, and stainless contact surfaces throughout the blast loop.
What actually causes the staining
Aluminium does not rust and stainless steel should not corrode in normal service. Both stain after blasting because fine iron particles are driven into the surface and stay there. Once the part is stored, handled or exposed to moisture, those particles corrode galvanically and the stain appears — often days later, which is why it is blamed on the coating.
The iron arrives by two routes: from the media itself, and from the machine. Cast steel shot, steel grit, cut wire shot and even 430 “stainless” shot carry enough iron to generate dust. Carbon-steel liners, pipes and separators wear in the same way. Blowing the part down does not remove embedded particles, and the contamination then shows up in powder coating, anodising, plating and passivation.
Media for aluminium parts
Aluminium is soft and takes embedded iron easily, so conventional cast steel shot, steel grit and high-iron abrasives are excluded. Use 304 stainless shot as the default, 316 where corrosion resistance matters, and glass bead or ceramic bead where the finish is the priority rather than cut.
| Media | Best for | Notes |
|---|---|---|
| 304 stainless shot | Aluminium, first choice | No blackening, no iron embedment, bright finish |
| 316 stainless shot | Marine, medical, food | Higher corrosion resistance |
| Aluminium shot | Supplementary | Gentle on the substrate, shorter life |
| Glass or ceramic bead | Precision, decorative | Fine finish, higher cost |
Size is 0.2–0.6 mm. For cosmetic, coating-ready or anodising work, a 0.2 + 0.3 mm mix gives the best balance of cutting power and surface fineness. Avoid 430 shot despite the name: its iron content still produces a grey, dull surface. Never mix carbon steel media with stainless media — one mixed batch contaminates the whole loop.
Media for stainless steel parts
Rust on stainless after blasting is a process problem, not a material defect. Use 304 for general work and 316 for chemical, seawater, food and pharmaceutical duty, normally 0.3–1.0 mm, adjusted to wall thickness and the roughness you need.
Stainless parts must run on dedicated equipment with dedicated media — never through a loop that also handles carbon steel. Before any changeover, clear the cabinet, hopper, belt, elevator, screw conveyor, separator and pipework of residual shot, broken media and iron dust. For valves, fittings and food-grade components, specify stainless liners, stainless media pipes and a stainless separator.
The machine must be stainless too
Changing media alone does not solve it. If the surfaces that touch the abrasive are still carbon steel, they wear and feed iron back into the circuit. For aluminium and stainless work, every surface in contact with media, workpieces or material flow should be 304 stainless.
- Chamber liners — 304 stainless wear plate instead of Mn13 (Hadfield manganese steel)
- Belt or track and buckets — stainless where the workpiece touches
- Screw conveyor — stainless auger and trough
- Elevator — stainless buckets, avoid iron debris in the return media
- Separator and media pipes — stainless throughout
- Storage and make-up hopper — stainless lining
This is the upgrade that makes the difference on a rubber belt tumble shot blasting machine or a hanger type machine running stainless work; the same specification applies to a CNC automated blasting cabinet. Stainless liners and pipework are ordered as stainless wear parts with the machine, not retrofitted later — request them in the media quote from our blast machine range.
Process discipline that keeps results stable
- Clean out before changeover. Chamber, hopper, belt, elevator, screw and separator, every time you switch to stainless media.
- Segregate media. Aluminium, stainless and carbon steel each need their own media and clear labelling.
- Screen and air-wash weekly. Broken shot, aluminium fines, iron dust and scale all degrade the finish.
- Control intensity. Soft aluminium needs lower wheel speed and shorter time; thin-wall parts need both reduced again.
- Protect parts immediately. A blasted surface is active — coat, passivate, oil or store dry without delay.
What to write on the specification
Three documents do most of the work. ISO 11124-6:2025 is the specification for stainless steel shot and irregulars used in blast cleaning: it sets particle size ranges, grade designations, hardness, density, metallographic structure and chemical composition. ISO 11125 gives the corresponding test methods. SAE J444 covers cast shot and grit size grading, and for peening work AMS 2431 is the usual reference.
Quoting ISO 11124-6 by grade and size removes the ambiguity that lets a supplier ship 430 and call it stainless. State the alloy (304 or 316), the size range, the standard reference and the application, and ask for the certificate with the delivery.
For the machine side, reference the coating or finish specification the part must meet and state that all media-contact surfaces are 304 stainless. Liner grade matters too: write 304 stainless wear plate, not simply “wear-resistant plate”, because Mn13 — the Hadfield manganese steel used as standard in carbon steel blasting — work-hardens well but sheds iron into the loop.
Quick selection guide
- Aluminium and die castings: 304 stainless shot, 0.2–0.6 mm; 0.2 + 0.3 mm mix for cosmetic parts
- Stainless pipes, valves, food machinery: 304 or 316 stainless shot, 0.3–1.0 mm
- Marine, medical, pharmaceutical: 316 throughout
- Machine configuration: 304 stainless wear plates and stainless media pipes on every contact surface
- Carbon steel work: cast steel shot or grit is correct — stainless media adds cost without benefit
The full range is on our shot blasting machines page, and for large fabrications rather than small parts a sand blasting room or a steel pipe blasting machine is the better starting point. Send the drawing and the finish specification through contact and we will confirm the media grade, the liner specification and the cycle.
Frequently asked questions
Can I use ordinary steel shot on aluminium parts?
No. Ordinary steel shot embeds iron in aluminium and starts galvanic corrosion at the surface. Use 304 or 316 stainless shot, or a non-ferrous media such as glass bead where cutting power is not required.
Why do stainless parts rust after blasting?
Iron contamination, almost always. The source is carbon steel media or worn carbon steel liners and pipes. Dedicated stainless media plus stainless contact surfaces removes both routes; cleaning alone does not stop it.
What size stainless shot should I use for aluminium?
0.2–0.6 mm. For appearance-critical, coating-ready or anodising parts, a 0.2 + 0.3 mm mix balances cleaning power against surface fineness better than a single size.
Can aluminium and stainless parts share one machine?
Yes, if that machine is fully stainless-lined and runs dedicated stainless media. Carbon steel parts must never share the same loop, because their dust contaminates both aluminium and stainless work.
Is 430 stainless shot acceptable?
Not for appearance or corrosion-critical work. It carries enough iron to produce a grey, dull surface and can still trigger staining. 304 or 316 is the correct choice.
How do I stop the problem coming back after a changeover?
Clean the entire loop, segregate and label the media, screen weekly, control intensity on soft parts and protect the surface immediately after blasting. Media choice alone is only half of the fix.
Which standard should stainless blasting media be quoted against?
ISO 11124-6:2025, metallic blast-cleaning abrasives, part 6: stainless steel (shot and irregulars). It sets particle size ranges, grade designations, hardness, density and chemical composition, with test methods in ISO 11125.
How do I write the media and liner requirement on an enquiry?
State the alloy (304 or 316), the size range and the grade to ISO 11124-6:2025, then add one line for the machine: all media-contact surfaces in 304 stainless wear plate, including separator and pipes. That removes both sources of ambiguity.
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