Plastic media blasting (PMB) was developed largely to solve exactly this problem. The governing specification for aerospace-grade blast media is MIL-DTL-85891C, which defines the qualified media types by resin chemistry and hardness. For composite coating removal, the relevant qualified types are Type II (urea formaldehyde) and Type V (acrylic). A third grade — Type VI (poly allyl diglycol carbonate / CR-39) — sits outside MIL-DTL-85891C but is still manufactured as a specialty grade and remains relevant for the most delicate work, so it is worth understanding alongside the two qualified types.
The three media types that matter for composites
Type II — Urea formaldehyde. A thermoset with a Mohs hardness around 3.5 (Barcol roughly 54–62). It is the workhorse of the plastic media family and the most widely used type across aerospace and general industry. On composites it is used where the coating is well-adhered and the laminate is robust — for example, certain carbon/epoxy structures. It offers the fastest strip rate of the three, at the cost of being the least forgiving.
Type V — Acrylic (PMMA). A thermoplastic, softer than urea (Barcol roughly 46–54) though still nominally Mohs ~3.5. Because it is gentler and considerably more forgiving in an operator's hands, Type V has become the preferred media for thin aluminum and for composite parts where substrate margin is thin. Where Type II strips aggressively, Type V trades some speed for a much wider safety window.
Type VI — Poly allyl diglycol carbonate (PADC / CR-39). The softest of the plastic grades (about 3.0 Mohs). Type VI sits outside MIL-DTL-85891C — it is not a qualified type under the current specification — but it is still manufactured as a specialty grade for the most delicate and critical work, including fiberglass, sensitive composites, and precision medical and instrument applications, because it preserves the surface profile of the part and does not mar or pit the substrate. When the priority is "do no harm," it remains the gentlest option, though it sits outside the military spec.
The common thread is a deliberately low hardness combined with an angular particle shape. The angularity gives the media genuine cutting action against a coating; the low hardness means it abrades the coating faster than it can abrade the laminate beneath. On a composite, where any dimensional change or fiber damage triggers an engineering review, that ratio is the whole game.
A practical note on qualification: authorization is substrate-specific. Programs such as NAVAIR have approved PMB for depainting certain carbon/epoxy and some metal/honeycomb structures, but have not blanket-approved it for fiberglass or specialized layups such as carbon-bismaleimide. Air Force use requires system-manager and engineering sign-off on the specific airframe. Plastic media is a tool that must be qualified to your part, not a universal solvent.
Why plastic media beats hard mineral abrasives
The reason glass bead, garnet, aluminum oxide, and silicon carbide are poor choices for composites comes down to a single hardness comparison:
| Media | Mohs hardness | Character |
|---|---|---|
| Type VI PADC (specialty, outside 85891C) | ~3.0 | Softest, gentlest on delicate parts |
| Type V acrylic | ~3.5 | Soft, angular, coating-selective |
| Type II urea | ~3.5 | Soft, angular, faster cut |
| Glass bead | 5–6 | Rounded, peening/polishing |
| Garnet | 7–8 | Sharp, aggressive profile |
| Aluminum oxide | 8–9 | Very sharp, fast-cutting |
| Silicon carbide | 9–9.5 | Hardest, fastest cut |
Every one of the mineral abrasives is dramatically harder than a carbon fiber or a resin matrix. On a composite, that means they don't just remove the coating — they cut reinforcing fibers and initiate delamination. The rule in the trade is blunt: for composites, fiberglass, and carbon fiber, use plastic blast media only, because any mineral abrasive will cut fibers and cause delamination.
Beyond fiber damage, hard media create three more problems that plastic avoids:
- Fatigue life. Aggressive angular abrasives, and even glass bead at pressure, can measurably reduce the fatigue life of thin structures — thin aluminum skins under ~0.080" are a documented example, and composite bond lines are equally sensitive. Plastic media's scrubbing action removes coating without imparting the deep anchor pattern or micro-cracking that shortens component life.
- Dimensional tolerance. Because plastic abrades the paint faster than the substrate, it removes coating with negligible loss of base material — critical where the part must hold tolerance or maintain its as-molded profile.
- Contamination and embedment. Mineral grit can fracture and embed in a soft surface, and iron-bearing media leave residue that promotes corrosion on adjacent metallics. Plastic leaves no silica or iron contamination behind.
There is an economic angle too: plastic at Mohs ~3–4 wears blast nozzles roughly two to three times slower than glass bead at Mohs 5.5–6.5, which adds up quickly in a production blast room running full shifts.
Why plastic media beats walnut shell and other organic media
Walnut shell, ground corn cob, and similar organics are soft, biodegradable, and genuinely gentle — which is why they get used on delicate substrates. But for coating removal on composites they fall short of plastic media in ways that matter:
- They don't reliably strip well-adhered aerospace coatings. Organic media are effective for cleaning, deflashing, and light contaminant removal, but they lack the consistent angular cutting geometry needed to lift a cured primer/topcoat system at a predictable rate.
- Inconsistent performance. Natural media vary in shape, size, and breakdown behavior, so strip rates and surface results are harder to standardize than with a mesh-controlled plastic abrasive held to a mil-spec.
- No qualification pathway. There is no MIL-DTL-85891C-equivalent traceability and approval infrastructure behind walnut shell for aerospace depainting, which is a non-starter for controlled programs.
Plastic media occupies the sweet spot: soft enough to protect the laminate, but engineered, sized, and specified tightly enough to strip on schedule and pass audit.
Why plastic media beats chemical stripping
Chemical stripping — historically methylene-chloride-based, now increasingly benzyl-alcohol or peroxide-activated formulations — was the traditional method for coating removal. Plastic media displaced much of it for good reasons, and the case is especially strong on composites:
- Matrix attack and absorption. Composites are porous and chemically active in a way metal is not. Aggressive strippers can be absorbed into the resin matrix, attack the polymer, swell the laminate, or degrade bond lines. A purely mechanical process removes coating without ever soaking the substrate in solvent.
- Layer selectivity and control. PMB lets an operator strip selectively — one coating layer at a time, or paint down to primer while leaving primer intact — and stop the instant the target is reached. Chemical strippers tend to attack the whole coating stack at once and are far harder to control on a delicate surface.
- Worker safety. Methylene chloride is a recognized health hazard subject to tightening regulation; even "safer" chemical strippers involve solvent exposure, ventilation demands, and PPE. Plastic media is non-toxic and, used with proper dust control, removes the solvent-exposure problem entirely.
- Waste stream. Chemical stripping generates hazardous liquid waste, solvent-laden sludge, and contaminated rinse water — expensive to handle and dispose of. Plastic media is recyclable: a single charge is reclaimed and reused many times until it breaks down to fines, dramatically shrinking waste volume and disposal cost. The spent stream is a small quantity of dry, largely inert plastic dust plus coating chips rather than drums of hazardous liquid.
- Speed and readiness. Blasting is a single-pass mechanical operation with no dwell time, no neutralizing, and no multi-cycle scrape-and-reapply. Parts come off the process dry and ready for inspection.
Honest limitations
Plastic media is not a cure-all. It is a line-of-sight process, so complex internal geometry and deep recesses are hard to reach. Media selection and blast parameters — pressure, standoff, angle, dwell — must be dialed in and controlled, and operator technique matters; over-blasting a single spot can still cause damage. Thermoplastic Type V floats and cannot be pumped in wet/slurry systems, so it is a dry-blast media only. And, as noted, authorization is airframe- and substrate-specific: some composites are simply not approved for PMB and must be handled another way. None of this undercuts the core advantage — it just means plastic media is a precision tool that rewards proper setup.
Selecting the right type
A reasonable default for composite work:
- Type V (acrylic) — the softer of the two MIL-DTL-85891C types, and the default for thin skins and composites needing a wide safety margin and a forgiving, gentle action.
- Type II (urea) — the other qualified type; use it where the laminate is robust and the coating well-adhered and you need the fastest qualified strip rate.
- Type VI (PADC / CR-39) — outside the military spec, but available as a specialty grade for the most delicate fiberglass, sensitive laminates, and precision work where profile preservation is paramount.
In every case, verify that the specific media/substrate combination is engineering-approved for your part, prove the process on a coupon before touching production hardware, and hold the media to MIL-DTL-85891C so results stay repeatable and auditable. Chosen and controlled properly, plastic media removes coatings from composites faster than chemistry, more safely than mineral grit, and more predictably than organics — which is exactly why it became the aerospace standard for the job.
AeroBlast can help you match the right grade and blast parameters to your substrate and coating system. Request a sample or call +1 513.772.4633 to discuss your application.