Unit 10 Comprehensive Quiz: Sandblasting & Surface Treatment

Unit: 10 - Sandblasting & Surface Treatment Duration: 30-45 minutes Passing Score: 70% Format: Multiple choice covering all modules Questions: 12

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Instructions

This comprehensive quiz covers all modules in the Sandblasting & Surface Treatment unit. You should complete all module assessments before attempting this unit quiz. The quiz tests both factual recall and application of concepts across modules.

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What is abrasive blasting (sandblasting), and what is its primary purpose?

Sandblasting uses high-pressure water to clean surfaces

Abrasive blasting propels abrasive media at high velocity against a surface using compressed air — it removes coatings, rust, scale, and contaminants, and creates a surface profile (texture) for coating adhesion

Sandblasting is a decorative technique that adds color to metal surfaces

Sandblasting uses chemicals to dissolve surface contaminants

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Explanation: Abrasive blasting (commonly called sandblasting, though actual sand is rarely used today due to silicosis risk) is a mechanical surface preparation method. Compressed air (60-120 PSI) accelerates abrasive particles (aluminum oxide, glass bead, garnet, steel grit, walnut shell, etc.) through a nozzle at high velocity. On impact, the particles remove surface material and create a roughened profile. Applications include: rust and paint removal, surface preparation before painting/coating, decorative etching on glass, deburring machined parts, and cleaning castings.

Why is actual silica sand no longer recommended as blasting media, and what has replaced it?

Sand is too expensive compared to alternatives

Sand doesn't work as well as other media for cleaning

Silica sand generates respirable crystalline silica dust that causes silicosis — an irreversible, potentially fatal lung disease — and has been largely replaced by aluminum oxide, garnet, glass bead, and other non-siliceous abrasives

Sand is banned worldwide by environmental regulations

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Explanation: Crystalline silica (quartz) fractures into microscopic particles (under 10 microns) during blasting that penetrate deep into the lungs. Chronic exposure causes silicosis — progressive, irreversible scarring of lung tissue. OSHA strictly limits silica exposure (PEL: 50 μg/m³). Safe alternatives: aluminum oxide (aggressive, reusable), garnet (moderate aggression, low dust), glass bead (gentle, for peening/polishing), walnut shell or corn cob (soft, for delicate surfaces), and steel grit/shot (aggressive, fully reusable, for steel). Each alternative matches or exceeds sand's performance for specific applications without the silicosis risk.

What PPE is required for abrasive blasting operations?

Safety glasses and work gloves are sufficient

A standard dust mask and hearing protection

Supplied-air blast helmet (or NIOSH-approved respirator with P100 filters minimum), blast suit or heavy leather protection covering all skin, hearing protection, and heavy-duty leather gloves — blasting generates hazardous dust, extreme noise, and high-velocity ricocheting particles

No PPE needed inside an enclosed blast cabinet

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Explanation: Abrasive blasting is one of the most PPE-intensive shop operations: (1) Respiratory — blast dust contains metal, coating debris, and media fragments. A supplied-air blast hood provides both clean breathing air and face/head protection. For blast cabinet work, a P100 respirator is minimum. (2) Hearing — blasting generates 100-120 dB, causing immediate hearing damage. (3) Body — ricocheting media impacts skin at high velocity, causing abrasion and embedding. Full-coverage blast suit or leather apron/sleeves required. (4) Hands — heavy leather blast gloves, not cotton or nitrile. Even inside a blast cabinet, respiratory protection is recommended due to dust leakage.

What is "surface profile" and why is it important for coating adhesion?

Surface profile is the color of the surface after blasting

Surface profile is how flat the surface is after treatment

Surface profile (also called anchor pattern) is the microscopic peak-and-valley texture created by abrasive impact — it provides mechanical "teeth" for paint, powder coating, or plating to grip, dramatically improving adhesion compared to a smooth surface

Surface profile only matters for decorative applications

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Explanation: Coatings adhere to surfaces through both chemical bonding and mechanical interlocking. Surface profile provides the mechanical component — microscopic peaks and valleys (measured in mils or microns) that the coating flows into and locks against. Profile depth depends on media type/size and blast pressure: aluminum oxide creates aggressive profiles (2-4 mil), glass bead creates finer profiles (1-2 mil). Coating specifications typically require a specific profile range — too shallow = poor adhesion, too deep = coating doesn't fully fill the valleys (creating voids). Profile is measured with a profile gauge or replica tape.

What is the difference between a blast cabinet and open blasting, and when would you use each?

There is no difference — they produce the same results

Blast cabinets are for metal only; open blasting is for wood only

A blast cabinet is an enclosed box with glove ports for small parts — it contains media and dust for reuse and safety. Open blasting (blast room/outdoor) handles large parts that don't fit in a cabinet but requires more PPE, media recovery, and environmental controls

Blast cabinets use air pressure; open blasting uses water

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Explanation: Blast cabinets (benchtop or floor-standing): enclosed work area with rubber glove ports, internal lighting, dust collector, and media recycling. Ideal for parts up to ~24" and production work. Advantages: contained media (reuse), minimal PPE (gloves built in), clean workspace. Open blasting (blast room, outdoor, field work): large nozzle + full PPE for oversized parts, structural steel, vehicles, or field work. Requires: blast hood/suit, media recovery (vacuum or sweep), environmental permits for outdoor work, and containment to prevent media/debris from spreading. Most educational makerspaces use blast cabinets exclusively.

How does media size (grit) affect the blasting result?

Larger media cleans faster and produces a smoother finish

Media size has no effect on the result — only pressure matters

Larger/coarser media (lower grit number) removes material faster and creates a rougher profile; finer media (higher grit number) produces a smoother finish with less material removal — choose based on the required surface finish and coating specification

Finer media removes material faster because it has more cutting edges

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Explanation: Media grit selection follows the same logic as sandpaper: (1) Coarse (24-60 grit) — aggressive removal of heavy rust, thick coatings, and mill scale. Creates deep profile (3-5 mil). Risk of substrate damage on thin material. (2) Medium (80-120 grit) — general purpose cleaning and surface prep. Profile 2-3 mil. Balanced aggression and finish. (3) Fine (150-320 grit) — light cleaning, deburring, cosmetic finishing. Profile 1-2 mil. (4) Very fine (glass bead 170-325 mesh) — peening, polishing, and satin finishing without significant material removal. Match media grit to the job: heavy rust needs coarse; aluminum cosmetic parts need fine.

What is "glass bead peening" and how does it differ from cleaning/stripping blasts?

Glass bead peening is the same as cleaning — it just uses glass instead of metal media

Glass bead peening applies a glass coating to the surface

Glass bead peening uses spherical glass beads at controlled pressure to create uniform compressive stress on the surface — this improves fatigue resistance and produces a uniform satin finish WITHOUT removing significant base material

Glass bead peening is only used on automotive body panels

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Explanation: Peening is fundamentally different from cleaning/stripping: the goal is to plastically deform the surface layer, not remove material. Each glass bead impact creates a tiny dimple, inducing compressive residual stress in the surface layer. This compressive stress resists fatigue crack initiation — the same principle as shot peening aerospace components. The cosmetic result is a uniform, non-directional satin texture. Applications: cosmetic finishing of stainless steel and aluminum, stress relief of machined parts, and blending tool marks. Glass beads are used at lower pressure (30-50 PSI) to avoid fracturing the beads.

What determines the correct blast pressure for a given job?

Always use maximum pressure for fastest results

Blast pressure should match the PSI rating on the media bag

Blast pressure depends on substrate material (hardness), media type, desired profile depth, and coating/contamination being removed — too high risks warping thin metal, embedding media, or removing base material; too low wastes time and produces inadequate profile

All blasting is done at a standard 80 PSI regardless of application

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Explanation: Pressure selection guidelines: (1) Soft substrates (aluminum, copper, wood): 30-50 PSI with fine media to avoid surface damage. (2) Mild steel general cleaning: 60-80 PSI with aluminum oxide or garnet. (3) Heavy rust/scale on thick steel: 80-100 PSI with coarse media. (4) Delicate work (glass etching, electronics): 15-30 PSI with fine media. (5) Sheet metal: moderate pressure (40-60 PSI) to avoid warping. Always start at lower pressure and increase until achieving the desired result. Testing on a scrap piece of the same material is essential.

What is "media contamination" and why must you prevent it?

Media contamination means the media has gotten wet

It refers to using too much media in the blast cabinet

Media contamination occurs when different abrasive types mix together in the blast system — this produces unpredictable results, can damage surfaces, and makes the media unsuitable for specific applications (e.g., ferrous media contaminating stainless steel surfaces)

Media contamination only happens with recycled media

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Explanation: Cross-contamination is a serious quality issue: (1) Ferrous contamination — steel grit/shot in a system also used on stainless steel or aluminum will deposit iron particles that cause rust staining on the non-ferrous surface. Stainless-contacting applications must use dedicated non-ferrous media in dedicated equipment. (2) Size mixing — coarse media mixed into a fine-media system creates random deep scratches in what should be a uniform fine finish. (3) Spent/broken media — crushed particles become dust that reduces visibility and creates irregular surface texture. Best practice: dedicate blast systems to specific media types and regularly screen media to remove broken particles.

How does abrasive blasting prepare a surface for powder coating specifically?

Blasting removes material so the powder coating fills in the missing thickness

Blasting heats the surface so the powder coating melts on contact

Blasting creates a clean, oxide-free surface with a controlled profile (1.5-3 mil) that allows the electrostatically charged powder particles to mechanically grip the surface, and the profile ensures adhesion after the powder melts and cures in the oven

Blasting is not needed before powder coating — the electrostatic charge is sufficient for adhesion

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Explanation: Powder coating adhesion depends on three factors that blasting provides: (1) Cleanliness — removes all oils, oxides, and contaminants that would prevent bonding. (2) Surface energy — freshly blasted metal has high surface energy (is chemically reactive), promoting molecular-level bonding with the cured coating. Surface energy degrades over time (oxidation returns within hours), so coating should follow blasting as quickly as possible. (3) Mechanical profile — the anchor pattern provides physical interlocking between the cured coating film and the substrate. Without adequate profile, powder coating can flake or chip under impact.

What environmental and housekeeping practices are required for safe blasting operations?

No special practices — blasting is a clean, contained process

Just sweep up visible media after each session

Continuous dust collection/ventilation during operation, immediate cleanup of spent media (slip hazard), proper disposal of contaminated media (may contain lead paint or heavy metals), and maintaining blast equipment (nozzle wear, hose inspection, moisture traps on air lines)

Environmental requirements only apply to outdoor blasting

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Explanation: Blasting housekeeping is both a safety and quality issue: (1) Dust control — fine particulate is a respiratory and explosion hazard; dust collectors must run during and after blasting. (2) Media on floors — spent abrasive is a severe slip hazard, especially on smooth concrete. Sweep or vacuum immediately. (3) Hazardous waste — media used to strip lead paint, chromium coatings, or cadmium plating becomes hazardous waste requiring special disposal. Test before disposing. (4) Equipment maintenance — worn nozzles increase media consumption by 30-50% and produce irregular blast patterns; inspect and replace at the first sign of bore widening. Moisture in air lines causes media clumping — drain the moisture trap before each session.

What is chemical etching, and when would you choose it over abrasive blasting?

Chemical etching and abrasive blasting produce identical results

Chemical etching is always preferred because it is safer than blasting

Chemical etching uses acid or alkaline solutions to dissolve surface material — it is preferred for intricate parts where blasting would damage fine features, for uniform treatment of complex geometries, and for preparing surfaces where blast media entrapment would be problematic

Chemical etching is only for glass, not metals

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Explanation: Chemical surface treatment (etching, phosphating, anodizing) uses controlled chemical reactions rather than mechanical impact. Advantages over blasting: (1) reaches internal surfaces and complex geometries that nozzles cannot access, (2) zero mechanical stress — no risk of warping thin sections, (3) uniform treatment regardless of geometry, (4) no media entrapment risk in blind holes or crevices. Disadvantages: chemical waste disposal costs, longer processing time, and less aggressive material removal. Common applications: phosphate conversion coating on steel (pre-paint), chromate conversion on aluminum, and acid etching of circuit boards. Many production workflows combine both: blast for heavy prep, then chemical treat for final surface conditioning.

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Last Updated: 2026-03-19