Choosing the right blasting media

Two operators with the same cabinet and the same pressure can get completely different results — one strips the surface clean in a single pass, the other polishes the paint shiny or digs into the base material. The difference is almost always the media.

Three properties decide: hardness (can it cut the coating without cutting the substrate?), shape (angular grains cut, round beads peen and smooth) and grain size (coarse removes fast and leaves a coarse profile, fine gives an even, smooth surface).

The rule of thumb is simple: choose media for the substrate, never for the coating alone. A media significantly harder than the substrate will mark it.

The name "sandblasting" has stuck, but ordinary sand has no place in any modern blasting setup. Quartz sand contains free crystalline silica, and the dust created when the grains shatter causes silicosis — a serious, incurable lung disease. In Norway it is therefore effectively prohibited to blast with sand containing more than one percent free quartz, and has been for decades.

All the media below are silica-free alternatives. The fact that crushed glass is made of silicates confuses some — but glass is amorphous, not crystalline, and does not release the dangerous quartz dust.

What it is: crushed, recycled glass, graded by grain size.

What it does: angular grains that cut efficiently through paint, rust and old coatings at moderate pressure, leaving a clean surface with a good anchor profile for new coating.

Suits: most things — bodywork, steel structures, trailers, rims, machine parts. If you are unsure and the substrate is steel, crushed glass is usually the right starting point.

Watch out for: the grains shatter on impact, so in open blasting it is effectively single-use. In a recycling cabinet it breaks down faster than aluminium oxide and garnet.

What it is: small, round beads of glass.

What it does: round beads do not cut — they peen. The result is an even, satin-matt surface with hardly any material removed — and the peening packs the surface slightly, an effect industry uses on purpose (shot peening) because it makes parts more resistant to fatigue. The beads survive many cycles in a recycling cabinet.

Suits: cleaning and finishing in a cabinet — engine parts, aluminium, stainless, tools. The default when the goal is an as-new look rather than heavy removal.

Watch out for: if progress is slow, the temptation is to raise the pressure — which just shatters the beads faster instead of working faster. If you need real removal, cut first with an angular media and finish with beads.

What it is: synthetic aluminium oxide — known in the trade as corundum — close to the top of the hardness scale among standard materials.

What it does: cuts fast and aggressively, leaves an etched profile, and survives a great many cabinet cycles before wearing out. That is why it is the workhorse in many blast cabinets.

Suits: hard coatings, mill scale, rust on solid steel, and keying surfaces before paint, coating or bonding. Where glass gives up, aluminium oxide keeps going.

Watch out for: too hard for thin panels, aluminium you want to preserve, and anything that cannot take marks. In soft metals, hard grains can also lodge in the surface — invisible embedment that later ruins paint or bonding, and the reason aluminium gets glass or soda instead. It puts a matt profile in most things — that is the point, but only when you want it.

What it is: a naturally occurring, dense mineral.

What it does: harder and denser than glass, so it cuts faster and gives an even, well-defined profile. Generates little dust, and can be recycled several times in closed systems. When a coating system specifies a blast profile, garnet is often what the datasheet assumes.

Suits: thick industrial coatings, mill scale and structural steel with profile requirements.

Watch out for: more expensive per kilo than glass, and too aggressive for thin sheet and soft substrates.

What it is: blast-grade sodium bicarbonate — coarse-grained "baking soda".

What it does: the crystals shatter on impact, and the shattering is what lifts the coating. Because soda is softer than aluminium, it physically cannot cut metal: the paint goes, the substrate stays untouched. It degreases as it works.

Suits: chrome, polished aluminium, thin panels, carburettors, and anything where the substrate is worth more than your time.

Watch out for: slow and single-use, and it leaves no anchor profile — if the surface is to be painted afterwards it needs separate keying. Soda is water-soluble, so a thorough rinse removes it completely — but it MUST happen before coating, and the runoff is alkaline — keep it away from plants.

Steel grit and steel shot are heavy industry's media: extremely durable and economical in large closed systems where they are recycled hundreds of times. They demand a completely dry system — moisture rusts both media and machine. Grit cuts, shot peens.

Walnut shell and other organic media are soft and gentle — wood, composites and light alloys clean up without a trace. They absorb moisture, so store the sacks dry; a clumped organic media has lost most of its effect. A bonus: the shells soak up oil and grease, so dirty engine parts degrease as they blast. Plastic media are used for stripping paint and powder coat where the substrate must not be touched at all — typically composites and parts going straight back into service.

The sack price lies about what a blasting media actually costs, because the media live very different lives: aluminium oxide survives a great many cabinet cycles — the grains fracture and expose fresh sharp edges instead of going blunt. Glass beads manage a good number of rounds at the right pressure. Crushed glass breaks down faster, and soda and organic media are in practice single-use.

So divide the sack price by the cycles the media realistically gives in your equipment, and the sums often flip: the most expensive media to buy can be the cheapest to run — and vice versa. In open blasting with no recovery everything is single-use, and cheap one-shot media like crushed glass win.

In a dry blast cabinet the media is recycled automatically, the dust is captured by extraction, and you choose the medium freely per job — glass beads for finish, aluminium oxide for removal. It is the most flexible workshop solution.

Wet blasting mixes the media with water. The water binds the dust, cushions the impact and produces a dense, even, characteristically fine surface — especially on aluminium and other light alloys. The process is gentler than dry blasting with the same media, and parts come out clean and degreased. The flip side: steel parts must be dried and protected promptly afterwards, or flash rust arrives before the coffee is poured.

Blasting is among the most air-hungry jobs in a workshop, and it is the nozzle that decides consumption — the larger the bore, the more air, and consumption climbs quickly as the nozzle wears. An undersized compressor means falling pressure, uneven results and long pauses.

So check the blasting equipment's stated air requirement against the compressor's actual delivery (FAD) at working pressure — not against the motor rating. And remember a water separator: moist air clumps the media and ruins the result.