Polishing & Blasting

Mechanical and abrasive processes that modify surface texture without adding a coating layer. Covers mirror polishing, hairline brushing, bead blasting, vibratory tumbling, sanding, and lapping. These processes remove material to achieve the target Ra -- the lower you go, the more it costs and the longer it takes. This guide covers which finish to pick, how much material to allow, and what goes wrong on the shop floor.

Which Finish Do You Need?

Most parts do not need a finish beyond as-machined. Only specify a finishing operation when there is a real functional or cosmetic requirement. Specifying mirror polish "because it looks nice" on internal components adds cost with zero benefit. Use this table to decide.

If Your Part Needs...	Finish	Typical Ra Target	Cost Factor
No cosmetic requirement, hidden or internal surface	As-machined	0.8–3.2 μm	Included
Uniform matte appearance, hide machining marks	Bead blast	1.6–6.3 μm	1x (baseline)
Uniform matte + consistent color before anodize or coating	Bead blast + anodize/coat	1.6–3.2 μm	1x + coating cost
Directional grain appearance (stainless panels, decorative trim)	Hairline / brush finish	0.4–1.6 μm	1.2–1.5x
Deburring and slight smoothing on many small parts at once	Vibratory tumbling	0.8–3.2 μm	0.5–1x (batch)
Smooth, semi-reflective surface for sealing or bearing contact	Fine grinding / sanding	0.2–0.8 μm	1.5–2x
Highly reflective or cosmetic (medical, consumer, food)	Polishing	0.1–0.2 μm	2–3x
True mirror reflection (optical, gauges, decorative)	Mirror polishing	0.05–0.1 μm	3–5x
Extremely smooth mating surfaces (gauges, precision fits)	Lapping	0.025–0.2 μm	4–6x

Rule of Thumb If the part is not visible to the end user and has no sealing, bearing, or friction requirement, leave it as-machined. Every finishing step beyond as-machined adds labor, lead time, and per-part cost. If you need appearance, bead blasting is almost always the cheapest way to get there. Mirror polish should only be specified when you genuinely need Ra 0.1 or better.

Finishing Methods at a Glance

Property	Mirror Polish	Hairline / Brush	Bead Blast	Vibro Tumble	Sanding / Grinding	Lapping
Ra Achievable	0.05–0.2 μm	0.4–1.6 μm	1.6–6.3 μm	0.8–3.2 μm	0.2–1.6 μm	0.025–0.2 μm
Surface Effect	Reflective to mirror	Directional lines	Uniform matte	Slightly smoothed, deburred	Smooth, directional (sanding) or flat (grinding)	Extremely smooth, flat
Cost Factor	3–5x	1.2–1.5x	1x	0.5–1x	1.5–2x	4–6x
Process Type	Manual, multi-stage	Semi-manual	Semi-automated	Batch, automated	Machine or manual	Machine, precision
Suitable Materials	SS, Al, brass, Cu, Ti	SS, Al	All metals	All metals, some plastics	All metals	Hard metals (steel, SS, carbide)
Dimensional Removal	0.01–0.1 mm	0.005–0.02 mm	Minimal (<0.005 mm)	0.01–0.05 mm	0.01–0.1 mm	0.005–0.05 mm
Lead Time Impact	+2–5 working days	+1–3 working days	+1–2 working days	+1–2 working days	+1–3 working days	+3–7 working days
Limitations	Labor-intensive, cannot reach deep pockets or internal bores	Directional -- only parallel lines	Cannot achieve smooth finishes, media embedment in soft metals	Limited control, not suitable for flatness or precision surfaces	Hard to access complex geometry	Flat surfaces or simple contours only, slow
Best For	Decorative, medical, optical, consumer	SS panels, architectural trim, consumer	Enclosures, brackets, any part needing uniform appearance	Deburring, edge-breaking, bulk small parts	Sealing faces, bearing seats, prep for polish	Gauges, valve seats, precision mating surfaces

Mirror Polishing Deep-Dive

Progressive abrasive process from coarse to fine grits, ending with buffing compound and (for true mirror) diamond paste or rouge. Each stage must completely remove the scratches from the previous stage -- if you skip a grit, the coarse scratches persist through the final polish and are visible under reflection. This is primarily a manual process, which is why it is expensive.

Process Stages

Stage	Abrasive / Tool	Ra Achieved	Result	Time (typical part)
1. Rough grind	80–120 grit belt or stone	1.6–3.2 μm	Removes major imperfections, deep tool marks, welds	5–15 min
2. Fine grind	240–400 grit belt	0.8–1.6 μm	Uniform direction, removes coarse scratch pattern	5–10 min
3. Pre-polish	600–800 grit or Scotch-Brite	0.4–0.8 μm	Semi-reflective, visible but fine texture	5–10 min
4. Final polish	Buffing wheel + cutting compound	0.1–0.2 μm	Highly reflective, haze-free	5–15 min
5. Mirror polish	Buffing wheel + rouge or diamond paste	0.05–0.1 μm	True mirror -- sharp reflection of objects	10–30 min

Achievable Ra by Material

Material	Best Achievable Ra	Difficulty	Notes
Stainless steel (304, 316)	0.05 μm	Moderate	Best candidate for mirror polish. Consistent result, no oxidation issues during polishing.
Aluminum (6061, 5052)	0.05 μm	Moderate	Polishes well but clogs abrasives faster. Soft -- easy to over-polish and create waviness. Requires frequent media changes.
Brass (C360, C260)	0.05 μm	Easy	Polishes quickly to mirror. Downside: tarnishes fast. Almost always needs clear lacquer or plating after polish.
Copper (C110)	0.05 μm	Easy	Polishes fast. Oxidizes quickly -- must be coated or plated immediately after polishing.
Titanium (Ti6Al4V, Gr2)	0.1 μm	Difficult	Hard and gummy. Work-hardens during polishing. Achieves 0.1 μm with effort. True mirror (0.05) is possible but slow and expensive.
Carbon steel (1045, 4140)	0.1 μm	Moderate	Polishes well but rusts immediately. Must be plated or coated after polish. Not practical as a final bare finish.
Zinc / Zamak die cast	0.4 μm	Poor	Too soft for aggressive abrasives. Smears instead of cutting. Limited to buffing only. Not suitable for mirror polish.

When to Specify Mirror Polish

Situation	Why Mirror Polish Is the Right Call
Medical / surgical instruments	Easy to sterilize, no crevices for bacterial contamination, visual inspection of cleanliness.
Food-processing equipment (contact surfaces)	FDA and food safety standards require smooth, cleanable surfaces. Ra 0.8 or better is typical minimum.
Optical housings and reflectors	Surface quality directly affects light reflection. True mirror (Ra 0.05) is required for critical optical applications.
Consumer products (high-end)	Brushed finish or mirror polish is the expected surface quality on premium consumer hardware.
Gauges and measuring faces	Smooth, flat surfaces are required for accurate measurement. Usually lapping is preferred over polishing.

When NOT to Specify Mirror Polish

Situation	Why Not
Internal surfaces, hidden cavities	Nobody sees it. You are paying for a finish that provides no value.
Parts that will be painted, coated, or plated after	The coating covers the polish. The polish is wasted effort. Bead blast or sanding is the correct prep for coating.
Parts with complex geometry (deep pockets, internal features)	Manual polishing cannot reach internal corners or deep recesses. You will get inconsistent finish -- mirror outside, rough inside.
Large flat surfaces (over 300 mm)	Hand-polishing large flat surfaces creates waviness and distortion. It is nearly impossible to hold flatness and achieve mirror Ra simultaneously without machine polishing.
Parts with tight dimensional tolerances	Polishing removes material. If tolerances are tight, polishing can push parts out of spec. Account for material removal in tolerance stack.

Polish Before Coating, Not After If the part will be anodized, plated, or coated, do the surface finishing (sanding, polishing) before the coating -- not after. The coating process (especially etching in anodize) will alter the surface. Polish to one grit level below your target, then let the coating process bring it the rest of the way.

Bead Blasting Deep-Dive

Abrasive media is propelled at high velocity (typically 40–100 psi) against the surface using compressed air. The impact creates a uniform matte texture by bombarding the surface with thousands of tiny impacts per second. This is the most cost-effective way to hide machining marks and create a consistent cosmetic finish. It works on any metal and on complex geometries -- the media reaches into pockets and recesses that manual finishing cannot access.

Media Types

Media	Composition	Result	Ra Achieved	Cost	Best For
Glass bead	Soda-lime glass, spherical	Smooth matte, satin sheen	1.6–3.2 μm	Low	Most common media. Cleans without heavy material removal. Good for aluminum, stainless, brass. Leaves no ferrous contamination.
Aluminum oxide (Al₂O₃)	Synthetic, angular	Coarser matte, etched surface	3.2–6.3 μm	Low	More aggressive than glass bead. Etches the surface, which improves coating adhesion. Standard media for surface prep before powder coat or painting.
Steel shot / grit	Cast steel, spherical or angular	Peened, smooth, slightly burnished	1.0–3.0 μm	Low	Shot peening for fatigue improvement. Leaves ferrous contamination -- not suitable for aluminum or stainless before anodize.
Ceramic bead	Zirconia or alumina, spherical	Fine matte, consistent	1.0–2.0 μm	Moderate	Reusable (lasts 20–50x longer than glass bead). Consistent finish. Good for production runs where finish uniformity matters.
Walnut shell / corn cob	Organic, soft	Very light cleaning, no surface alteration	No significant change	Low	Deburring and cleaning soft metals (aluminum, brass, copper) without removing material. Used for deflashing plastic parts.
Silicon carbide (SiC)	Synthetic, very angular, sharp	Aggressive etching, rough surface	3.2–12.5 μm	Low	Heavy cleaning, removing rust or scale, surface prep for thick coatings. Too aggressive for cosmetic finishes.
Plastic bead (PMA)	Urea-melamine or acrylic	Light cleaning, gentle stripping	Minimal change	Moderate	Stripping coatings without damaging the substrate. Used in aerospace for paint removal. No substrate damage.

Grit Sizes and Resulting Ra

Grit / Mesh Size	Particle Size	Typical Ra (glass bead)	Typical Ra (Al₂O₃)	Visual Result
80–120 mesh	125–180 μm	3.2–6.3 μm	6.3–12.5 μm	Coarse matte, visible texture. Aggressive cleaning.
150–200 mesh	75–106 μm	2.0–4.0 μm	3.2–6.3 μm	Medium matte. Standard for cosmetic blasting.
220–270 mesh	53–75 μm	1.6–2.5 μm	2.5–4.0 μm	Fine matte, satin-like. Common for anodize prep.
325–400 mesh	38–45 μm	1.0–1.6 μm	1.6–3.2 μm	Very fine matte, almost smooth. Good pre-polish step.

Masking for Selective Blasting

Blasting hits everything in the blast cabinet. If you need a machined surface (bearing seat, sealing face, tight-tolerance bore) to remain unblasted, you must mask it.

Masking Method	Best For	Cost	Notes
High-temp tape	Flat surfaces, simple selective areas	$0.30–1 per application	Quick to apply. Can peel at edges under blast pressure. Not precise.
Silicone plugs / caps	Bores, holes, tube ends	$0.50–3 per plug	Reusable. Precise. Best for protecting threaded holes and bores.
Custom fixtures / shields	Production runs, complex patterns	$50–300 tooling (one-time)	Amortized over volume. Fast to apply. Essential for production.
Photoresist / laser-cut vinyl	Logos, text, precise patterns	$2–10 per part	Can create logos or text by blasting through a stencil. Popular for branding on aluminum enclosures.

Media Contamination Do not use steel shot or steel grit on aluminum or stainless parts that will be anodized afterward. Ferrous contamination embeds in the surface and causes dark spots and discoloration during anodize. Use glass bead or ceramic bead on aluminum and stainless. If steel media must be used, follow with a glass bead blast to remove embedded ferrous particles.

Bead Blast + Anodize / Coating Bead blasting before anodizing or coating is standard practice. The blasting creates a uniform surface texture that the coating adheres to well, and it hides minor machining imperfections. For aluminum parts, glass bead at 220–270 mesh followed by Type II anodize is the most common cosmetic finish combination.

Vibratory Tumbling

Parts and abrasive media are placed in a vibrating bowl or tub. The vibration causes the media to slide, tumble, and rub against the parts, gradually deburring edges, smoothing surfaces, and breaking sharp corners. The process is batch-oriented -- you load a batch, run it for a set time, then unload. Labor per part is very low, making it the cheapest finishing option for high-volume small parts.

Media Types

Media	Composition	Aggressiveness	Result	Best For
Ceramic triangle	High-alumina ceramic	High	Deburring, surface smoothing, fast cut	Steel and stainless parts. Heavy burr removal.
Ceramic cylinder	High-alumina ceramic	High	Deburring, edge-breaking, smoothing	General purpose. Good reach into holes and slots.
Porcelain ball	Porcelain ceramic	Medium	Polishing, burnishing, fine smoothing	Pre-plate or pre-anodize finishing. Leaves a smooth, burnished surface.
Plastic pyramid	Urea or polyester	Low	Light deburring, no dimensional change	Soft metals (aluminum, brass, copper). Prevents media embedding in soft substrates.
Steel ball	Hardened steel	Low	Burnishing, peening, surface compaction	Improves surface fatigue life. Leaves a shiny, burnished finish.
Dry organic (corn cob, walnut)	Natural, organic	Very low	Drying, light cleaning, polishing	Final-stage drying after wet tumbling. Light polishing of soft metals.

Cycle Times and Batch Processing

Operation	Media	Typical Cycle	Compound	Result
Heavy deburring	Ceramic triangle, large	2–6 hours	Deburring compound	Removes burrs up to 1 mm. Significant edge break.
Light deburring	Ceramic triangle, small	1–3 hours	General-purpose compound	Removes small burrs, breaks sharp edges. Minor surface smoothing.
Surface smoothing	Porcelain ball	2–4 hours	Burnishing compound	Smooths surface to Ra 0.8–1.6. Prep for plating or anodize.
Polishing	Porcelain ball + polishing compound	4–8 hours	Polishing paste	Semi-reflective finish. Not mirror, but visibly polished.
Drying	Corn cob or walnut shell	30–60 min	Dry (no liquid)	Removes moisture from wet tumbling. Parts come out dry.

Suitable Part Types

Good Fit	Poor Fit	Why
Small machined parts (under 100 mm)	Large parts, thin-wall, precision	Parts must fit in the bowl (typically 300–600 mm diameter). Parts can dent or deform each other in the batch. Tumbling is non-selective -- all surfaces get finished.
CNC turned and milled components
Stampings and castings
Screws, bolts, small brackets
Parts that need deburring on all edges	Parts with tight tolerances on specific faces	Cannot protect individual surfaces from the media. All surfaces are processed.
High-volume runs (50+ pcs)	Prototypes or one-off parts	Batch setup takes time. For 1–5 parts, hand deburring is faster and cheaper.
Parts with uniform material thickness	Sheet metal, thin-wall tubes, delicate features	Part-on-part impact in the bowl can dent thin material or bend delicate features.

Part-on-Part Damage In a vibratory batch, parts rub against each other. On aluminum and brass, this causes dents and scratches. Add a separator (divider or pins) between parts, or use a media-to-part ratio of at least 5:1 to cushion the impact. For delicate parts, consider barrel tumbling with a higher liquid level or hand finishing instead.

Ra Values and What They Mean

Ra (arithmetic average roughness) is the most common surface roughness parameter. Measured in micrometers (μm) or micro-inches (μin). It measures the average deviation of the surface profile from the mean line. Lower Ra = smoother surface = more expensive. Most engineering drawings reference Ra as the surface finish callout.

Ra (μm)	Ra (μin)	Visual Description	How It Feels	Cost Multiplier (vs as-machined)	Typical Application	Achievable By
0.05	2	True mirror -- sharp, undistorted reflection of objects	Perfectly smooth, glass-like	4–6x	Optical mirrors, precision gauges, high-end decorative	Mirror polishing (diamond paste/rouge)
0.1	4	Very smooth, hazy reflection -- objects are visible but blurred	Very smooth, barely detectable texture	3–4x	Medical instruments, cosmetic consumer products, sanitary surfaces	Mirror polishing
0.2	8	Semi-reflective, faint lines visible under close inspection	Smooth, very slight texture detectable with fingernail	2–3x	Bearing surfaces, hydraulic cylinder bores, seal faces	Fine polishing, fine grinding
0.4	16	Smooth, fine directional lines visible. Low reflectivity.	Smooth, slight texture under fingertip	1.5–2x	General precision mating surfaces, pump components	Fine grinding, pre-polish, lapping
0.8	32	Smooth but machining marks visible on close inspection	Even texture, slight roughness	1.2–1.5x	CNC finishing pass, light mating surfaces, general machined	CNC finish pass, vibro tumble
1.6	63	Visible tool marks. Standard machined appearance.	Clearly rough, machining marks felt	1x	Non-critical machined faces, standard CNC output	CNC standard, bead blast (fine)
3.2	125	Rougher, distinct machining lines. Common on roughing passes.	Rough, clearly felt	Included	Non-critical surfaces, hidden faces, roughing pass	CNC roughing, bead blast (coarse)
6.3	250	Clearly rough, deep machining marks, saw-cut appearance	Very rough, catches fingernail	Included	Hidden surfaces, pre-coating, cast surfaces	Sawing, coarse grinding, heavy blasting

Specify Ra, Not "Smooth" or "Polish" "Smooth" and "polish" are not measurable. Different shops interpret them differently, and you have no basis for rejecting parts. Always specify a numerical Ra value on the drawing: "SURFACE FINISH Ra 0.8" or "Ra 1.6 MAX." If you need a specific process, call it out: "MIRROR POLISH Ra 0.1" or "BEAD BLAST Ra 3.2."

Dimensional Impact

Every mechanical finishing process removes material. The amount removed depends on the process, the starting surface condition, and how aggressively it is applied. Unlike coatings (which add material), finishing operations make your part smaller. This matters for any feature with a tolerance tighter than ±0.01 in.

Process	Material Removal Per Surface	Removal on a Diameter (both sides)	Can You Control It?	Recommendation
As-machined	0 mm	0 mm	N/A	No allowance needed.
Bead blasting	0.002–0.005 mm	0.004–0.01 mm	Limited -- depends on media, pressure, time	Negligible for most tolerances. Mask tight features.
Hairline / brush	0.005–0.02 mm	0.01–0.04 mm	Moderate -- controlled by grit and passes	Allow 0.02 mm on brushed faces. Not suitable for tight-tolerance surfaces.
Vibratory tumbling	0.01–0.05 mm	0.02–0.1 mm	Low -- batch process, hard to control precisely	Allow 0.02–0.05 mm overall. Do not tumble parts with tight tolerances.
Sanding / grinding	0.01–0.1 mm	0.02–0.2 mm	Good -- controlled by grit progression	Allow 0.02–0.05 mm for fine grinding. More for rough grind.
Polishing	0.01–0.1 mm	0.02–0.2 mm	Moderate -- depends on starting surface	Allow 0.02–0.05 mm for polish from machined surface. More if heavy grinding is needed first.
Mirror polishing	0.02–0.1 mm	0.04–0.2 mm	Moderate -- multi-stage, cumulative	Allow 0.05 mm minimum on mirror-polished surfaces. Starting from a good machined finish reduces removal.
Lapping	0.005–0.05 mm	0.01–0.1 mm	Very good -- controlled by time and abrasive	Lapping is often used to achieve final dimension. Allow 0.01–0.03 mm for lapping stock.

Feature Type	Risk Level	Recommendation
Press-fit bores (H7/p6)	High	Mask or do not finish these surfaces. Even 0.01 mm removal changes the interference fit.
Threaded holes	Medium	Bead blasting is usually fine (minimal removal). Polishing removes thread peaks -- may cause loose fit. Mask or avoid.
Shaft diameters in bearings	High	Do not polish or blast bearing journals unless you account for the removal in tolerance stack.
Sealing surfaces (O-ring grooves)	Medium	Sanding or lapping sealing faces is fine if controlled. Avoid blasting (media embedment).
Flatness-critical faces	High	Hand polishing creates waviness on large flat surfaces. Use machine grinding or lapping for flatness requirements.
External cosmetic surfaces	Low	Dimensional removal is acceptable on appearance surfaces as long as it is within profile tolerance.

Cost Drivers

Mechanical finishing costs are driven almost entirely by labor. Unlike coating or plating (where material cost matters), polishing and blasting are about how long a person or machine works on each part. Understanding what drives the time helps you reduce cost without compromising the result.

Cost Factor	Impact	Detail
Labor (manual polishing)	Dominant cost	Mirror polishing is 80–100% hand labor. A skilled polisher charges $15–40/hr. A complex part with internal features can take 30–60 minutes per surface. This is why mirror polish is expensive.
Surface area	High	Cost scales with area. A 50×50 mm flat plate is cheap. A 500×500 mm plate with the same Ra takes 100x longer. Consider whether every surface needs the finish.
Number of surfaces	Moderate to high	Each additional surface adds time. A 6-sided box finishing on all faces costs roughly 6x a single-face finish. Minimize the number of surfaces that need finishing.
Geometry complexity	Moderate to high	Internal corners, deep pockets, and undercut features require hand tools and slow work. Simple flat surfaces and external radii are fastest.
Starting surface condition	Moderate	If the machined surface is already Ra 0.8, polishing to Ra 0.2 takes less time than starting from Ra 3.2. A good CNC finishing pass before polishing saves money.
Setup and masking	$2–8 per part	Masking surfaces that must not be finished adds labor for application and removal. Custom fixtures amortize at volume.
Quantity breaks	Significant	Bead blast: minimal quantity effect (quick per part). Polishing: 10–20% cheaper at 50+ pcs as operators develop efficiency. Vibro tumble: large quantity effect -- 100 pcs costs barely more than 10 pcs.
Ra target	Non-linear	Cost does not scale linearly with Ra. Going from Ra 1.6 to Ra 0.8 costs 1.5x. Going from Ra 0.2 to Ra 0.05 costs 3–4x. The last few tenths of a micron are exponentially more expensive.
Bead blasting setup	$30–100 lot charge	Most blast shops have a minimum charge. For 5 parts, setup dominates. For 500 parts, per-part cost is minimal.
Rush / expedite	+25–100%	Standard lead time: 2–5 working days. Rush disrupts schedules. Skilled polishers may not be available on short notice.

Reducing Polishing Cost Three practical ways to cut polishing cost: (1) specify a CNC finishing pass with a ball-nose end mill or fly cutter before polishing -- starting from Ra 0.8 instead of Ra 3.2 cuts polishing time by 50% or more; (2) limit mirror polish to visible surfaces only, leave hidden faces as-machined; (3) design with large, simple external radii instead of tight internal corners -- flat external surfaces polish 5–10x faster than internal pockets.

Common Mistakes

Mistake	Consequence	Fix
Specifying mirror polish on parts that will be painted or coated	Wasted money. The coating covers the polish completely. Ra 0.05 mirror under 60 μm powder coat is invisible.	Use bead blast or sanding as coating prep. Only polish if the final surface is bare metal.
Not accounting for material removal on tight-tolerance features	Bearing journals too small, bores too large, sealing faces out of flat. Parts fail dimensional inspection after finishing.	Add finishing stock to your tolerance stack. For polishing, allow 0.02–0.05 mm. For grinding, 0.01–0.05 mm. Mask critical features.
Specifying Ra without a process or reference standard	Shop interprets Ra 0.4 differently from what you intended. One shop delivers a ground surface, another delivers a buffed surface. Both measure Ra 0.4 but look different.	Call out the process: "Ra 0.4, GROUND" or "Ra 0.4, POLISHED" or reference a standard (ISO 1302, ASME Y14.36). Include a surface finish symbol on the drawing.
Using steel media on aluminum before anodize	Ferrous particles embed in the aluminum surface. During anodize etching, these particles cause dark spots and discoloration that are visible through the coating.	Use glass bead or ceramic bead on aluminum and stainless parts. Never use steel shot or grit on parts that will be anodized.
Mirror polishing large flat surfaces by hand	Waviness and distortion. The surface reflects but the reflection is wavy and distorted. Flatness is lost.	Use machine lapping or surface grinding for large flat areas. Reserve hand polishing for contoured and external surfaces.
Specifying the same Ra on all surfaces	Unnecessary cost on hidden and non-critical surfaces. If you call out Ra 0.8 on all faces, the shop finishes everything -- including surfaces nobody will see.	Call out Ra only on the surfaces that need it. Leave the rest as-machined or use a blanket note: "UNLESS OTHERWISE SPECIFIED, SURFACE FINISH Ra 3.2."
Polishing stainless steel and not protecting it	Fingerprints, water spots, and light surface rust appear within days. Mirror-polished stainless shows every mark.	Apply a clear coat, passivate (for corrosion resistance), or package in protective film. For outdoor use, consider electropolishing instead of mechanical polishing.
Tumbling thin-wall or delicate parts without separators	Parts dent, bend, or scratch each other in the vibratory bowl. High scrap rate.	Use part separators, increase media-to-part ratio to 8:1 or higher, or switch to hand finishing for delicate parts.
Not specifying blast media type or grit size	Shop uses their default, which may be too aggressive (coarse Al₂O₃ instead of fine glass bead). Surface is rougher than expected, or contaminated with wrong media.	Specify on the drawing: "GLASS BEAD BLAST, 220 MESH, Ra 2.0 MAX" or equivalent. Include media type and grit.
Skipping the CNC finishing pass and sending rough-machined parts to polishing	Polishing time (and cost) triples or more. The polisher must remove all the rough tool marks before even starting the polish sequence.	Request a CNC finishing pass (ball-nose mill, fly cutter, or fine boring) to Ra 0.8–1.6 before polishing. The small extra CNC cost saves significant polishing labor.

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