Guide
How should a drawing call out masking for anodizing on bores, threads and contact surfaces?
Masking for anodizing is the practice of covering selected surfaces of an aluminum part with plugs, caps, tape or a liquid maskant before it goes into the anodizing tank, so those surfaces stay bare metal while the rest of the part is coated. The drawing decides which surfaces are masked, what protects them afterward, and how the boundary between coated and bare metal is judged.
By Saola Manufacturing engineering team · Published · Updated
This guide is for design engineers and sourcing managers who release anodized parts to a supplier, including one overseas. It covers precision bores, conductive surfaces and tapped holes, and draws on the two military specifications most US drawings cite for this work, the anodic coating specification MIL-A-8625F and the chemical conversion coating specification MIL-DTL-5541. Where this page quotes a figure, it is the figure published in those documents, not a statement about any particular finisher.
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When should a bore or threaded hole be masked instead of anodized?
Mask a feature when the coating would break its function: a bore or seat that must hold a size the coating would change, a face that must carry current or ground the part, or a surface that will be welded, bonded or pressed after finishing. Everything else is normally better off coated, because an anodized surface resists corrosion and wear and a bare one does not.
Anodic coatings grow into and out of the metal at the same time, so every coated surface moves. Per MIL-A-8625F section 6.10.1, a surface can be expected to grow by about one half of the coating thickness, and the specification gives the example that a coating of 0.004 inch calls for a pre-machining allowance of 0.002 inch per surface on close-tolerance parts (MIL-A-8625F). A bore loses twice that growth on its diameter, which can consume a tight fit's whole band.
There are two ways to keep such a bore on size. The first is to machine it undersize, anodize it, and let the coating bring it to size, which works when the finisher can hold the coating thickness closely and the bore benefits from a hard wear surface. The second is to mask the bore and leave it bare. Masking is safer when the fit is too tight to trust coating variation or a bearing race should seat on metal. A third option, covered below, is to coat the bore and then machine or hone the coating away.
Electrical contact is the other common reason. Anodic oxide is an electrical insulator, which is why MIL-DTL-5541F describes its Class 3 conversion coating as the choice where lower contact resistance than an anodic coating is needed. Grounding lugs, EMI gasket lands, battery contacts and enclosure seams that must bond to the next part all need a masked, conductive surface.
What do MIL-A-8625 and MIL-DTL-5541 say about holes, masked areas and contact marks?
Neither specification tells you which features to mask; that is left to the drawing and the purchase order. What they do give is the design guidance and the rules that shape a good masking note, and the table below collects the clauses that matter most.
| Topic | Clause | What it says | What it means for the drawing |
|---|---|---|---|
| Growth on each surface | MIL-A-8625F 6.10.1 | About half the coating thickness builds up on each coated surface; place both a machining and a coated dimension on the drawing | Say whether a bore dimension applies before or after anodizing, or mask it |
| Small and tapped holes | MIL-A-8625F 6.10.1.1 | Coating inside small and tapped holes can vary from no film to a full coating; holes over 1/4 inch should be anodized | Do not count on coating in small holes for corrosion protection; state whether small holes may stay uncoated |
| Threads | MIL-A-8625F 6.10.2 | Pitch diameter of a 60° thread grows by four times the surface growth | Mask the thread, or tap oversize and gauge after coating |
| Sized after coating | MIL-A-8625F 6.7 and 6.10.1.1 | Type III may be processed oversize and lapped or honed; Type II surfaces with a total tolerance of 0.0004 inch or less, if sized after anodizing, should be treated with QPL-81706 materials | If a bore is honed through the coating, call out a conversion coating on the exposed metal |
| Contact marks | MIL-A-8625F 3.13.1 and 6.2 | Contact marks are kept to a minimum; if their location matters, it is specified on the order | Name where rack contacts are allowed, often inside a masked area |
| Touch-up | MIL-A-8625F 3.3.4; MIL-DTL-5541F 3.4 | Mechanically damaged areas may be touched up with QPL-81706 conversion materials, limited to 5 percent of the area | Touch-up repairs damage; it is not a substitute for a masking callout |
| Conductive areas | MIL-DTL-5541F 1.2 and 6.1.2 | Class 3 is for corrosion protection where low electrical resistance is required; it is thinner and less protective than Class 1A | Call out Class 3 on masked areas that must conduct |
Two further clauses are worth knowing. MIL-A-8625F section 3.3.1.1 says parts are anodized after machining, heat treatment and forming, so any operation that cuts through the coating afterward needs to be written on the drawing as an exception. And section 6.2.2 lets the supplier choose the type and class when the drawing leaves them out, so a masking note attached to “anodize” with no type is only half a specification.
Should masked areas be left bare or given a chem film?
Leave a masked area bare only if it is sealed inside an assembly or protected some other way; otherwise call out a chemical conversion coating on it, Class 3 per MIL-DTL-5541 where the area must conduct and Class 1A where it only needs corrosion protection and paint adhesion.
Bare aluminum is poorly protected next to the anodized surface around it. A conversion coating, often called chem film or by trade names, adds corrosion resistance without adding measurable thickness, so it suits bores and faces that were masked for dimensional reasons. MIL-DTL-5541F section 6.1 describes these coatings as intended for aluminum that is not anodized and for repairing anodized coatings, which is exactly the situation of a masked area on an anodized part.
The Class 3 electrical figure is set by the qualification specification. Per MIL-DTL-5541F section 6.1.2.1, Class 3 materials are qualified under MIL-DTL-81706 at a contact resistance not greater than 5 milliohms per square inch as supplied (the specification states it as five thousand microhms), measured at a nominal electrode pressure of 200 psi. The same section warns that surface roughness and electrode condition can affect the reading more than coating thickness does, so if resistance on the part matters, specify the value and the test on the order rather than assuming the qualification number carries over.
Conversion coating normally goes on the masked areas after anodizing and mask removal, since it acts on bare aluminum. Write the sequence down if more than one supplier is involved.
How do you write a masking note on the drawing?
Show the masked surfaces graphically, with a chain line or a shaded zone tied to a flag note, and write the note so it names the coating specification, the masked features, what they receive instead, and where rack contacts may fall. A finisher who never saw your model should be able to mask the part from the PDF alone.
ASME Y14.5-2018 provides the chain line for indicating a limited area or length of a surface that carries a special requirement, and it is the cleanest way to bound a masked face on an orthographic view. For a bore, a flag note pointing at the bore is enough. The notes below are starting templates; adjust the types and classes to your part.
- Masked bore. “ANODIZE PER MIL-A-8625, TYPE II, CLASS 1. MASK BORE ØA FULL DEPTH AND FACE B; NO ANODIC COATING PERMITTED IN MASKED AREAS. CHEMICAL CONVERSION COAT MASKED AREAS PER MIL-DTL-5541, TYPE II, CLASS 1A AFTER ANODIZE.”
- Grounding or bonding face. “MASK SURFACES INDICATED BY CHAIN LINE. CHEMICAL CONVERSION COAT MASKED SURFACES PER MIL-DTL-5541, TYPE II, CLASS 3. SURFACES MUST BE ELECTRICALLY CONDUCTIVE AFTER FINISH.”
- Tapped holes. Either “MASK ALL TAPPED HOLES; PLUGS MAY EXTEND TO THE COUNTERSINK” or “TAPPED HOLES MAY BE COATED; THREADS TO GAUGE AFTER FINISH.” Pick one; leaving it unstated invites a supplier choice.
- Dimensions. “ALL DIMENSIONS APPLY AFTER FINISH EXCEPT MASKED FEATURES, WHICH APPLY TO BARE METAL.”
- Rack contacts. “RACK CONTACT MARKS PERMITTED ONLY IN MASKED AREAS OR ON SURFACE C.” This uses the option MIL-A-8625F gives the buyer to specify contact-mark location.
Also say how the edge of the masked area is judged. Masking boundaries are never perfectly sharp, so dimension the boundary from a datum and state which side may carry a slight overlap: coating allowed to run a short way into the masked zone, or bare metal allowed to show a short way outside it. If it does not matter functionally, say so. For how the growth itself is calculated, the finish comparison guide covers anodizing thickness by type and its effect on dimensions.
Which masking methods suit bores, threads and flat faces?
Round features take plugs and caps, flat or irregular faces take tape or a brushed or dipped liquid maskant, and blind or very small holes are often better left to coat and be sized afterward. The method is the finisher’s choice, but the geometry you draw decides how clean the result can be.
- Plugs and caps. Tapered or stepped plugs seal a through bore or a tapped hole quickly and can be reused. They work best when bores follow standard sizes and have a chamfer or edge where the plug seats.
- Tape. Chemical-resistant tapes cover flat lands and grounding pads. Straight boundaries are easy; ones that wrap a radius are not.
- Liquid maskant. Painted-on or dipped lacquers cover irregular shapes. They take longer to apply and strip, and the edge is only as sharp as the hand that applies it.
Design helps more than any masking material. Put a small step, groove or chamfer where coated and bare meet, and the maskant has a physical edge to stop at, which hides any unevenness. MIL-A-8625F section 6.10.3 also asks for edges and inside corners to be radiused before anodizing, because the coating develops voids at sharp corners, and that advice applies equally to the edge of a masked bore.
How does masking change the cost of an anodized part?
Masking is manual work applied part by part, so its cost scales with the number of masked features, how hard each is to reach, and how tight the boundary tolerance is. A part with a few plugged bores costs little extra; a part with many irregular masked faces and a tight boundary can cost more to mask than to anodize.
The main cost drivers are easy to control at the design stage:
- Feature count. Mask only what the function needs. Tapped holes that take ordinary fasteners can often be coated and gauged instead of masked one by one.
- Boundary tolerance. A loose or functionally irrelevant boundary lets the finisher work quickly. A boundary dimensioned tightly on a flat face is slow to apply and slow to inspect.
- Post-coat machining as the alternative. On a part already going back to a machine for other features, cutting the coating off a bore in the same setup can cost less than masking it. On a part that would otherwise leave the finisher complete, masking usually wins.
Die-cast housings add one more factor. MIL-A-8625F section 6.16 states that a hardcoat of 2 mils (0.002 in) or more is extremely difficult to obtain on high-silicon die castings such as 360, 380 and 383. If a drawing calls for thick hardcoat on such a housing, confirm with the finisher what thickness is realistic before sizing bore allowances, rather than assuming the nominal callout. The ADC12 and A380 alloy guide explains why these alloys behave differently in finishing.
How do you make sure an overseas supplier masks the right features?
Put every masking instruction on the drawing and in the purchase order, not in an email, and ask for a first article with the masked features checked and recorded. The finishing line reads the print; anything that is not on it is at risk of being lost.
This matters most when machining and anodizing happen at different companies, which is common whether the parts come from Vietnam, Mexico or a domestic job shop. The anodizer sees only the part and the paperwork that travels with it.
For the first article, ask for three things: the masked bores measured after finishing, not before; a simple electrical continuity check on every surface that must conduct, since an anodized surface will read as open; and photographs of the masking boundaries against the drawing. Those quick checks catch the usual failures: a slipped plug, a missed note, a coated grounding pad.
When finishing is part of the same order as CNC turning or milling, send the masking requirement with the RFQ so it is priced from the start; the RFQ package guide lists what else belongs in that package. The surface finishing page lists the finishes our partner plants apply, and the quote names where each step is done; a drawing with masking notes can be sent for a quote, where any feature that needs a masking decision is raised in the reply.
Frequently asked
Can a hard-anodized bore be honed to size instead of masked?
Yes. MIL-A-8625F notes that Type III coatings may be processed oversize and then lapped or honed to the final dimension. The bore keeps a hard, wear-resistant surface, but honing takes time and needs a finisher or machinist who can measure through the coating.
Do I need to mask tapped holes for Type II anodizing?
Not always. Coating in small tapped holes is often thin or patchy, and many ordinary fastener threads still gauge after a thin coating. For precise threads, or thicker Type III coatings, mask them or tap oversize and gauge after finish. The rule is to state which on the drawing.
Is chem film enough protection for a masked bore?
For a bore that sits inside an assembly with a bearing or shaft in it, a conversion coating is usually adequate. For a surface exposed to weather or salt, it is less protective than anodizing, and Class 3 is thinner and less protective than Class 1A, so choose the class by function.
What happens if the drawing just says “anodize” with no masking note?
The whole part may be coated, including bores and grounding faces, and MIL-A-8625F lets the supplier pick the type and class if you did not. Stating the type, class and every masked feature removes those choices from the supplier.
Can masked areas be dyed to match the rest of the part?
No. Dye is absorbed by the porous anodic coating, and a masked area has none, so it stays the color of bare or conversion-coated aluminum. If appearance matters, keep masked areas on hidden or functional faces.
Sources
- MIL-A-8625F, Anodic Coatings for Aluminum and Aluminum Alloys, US Department of Defense, 1993, with Amendment 1 (2003) and a 2019 notice of validation, both still titled MIL-A-8625F; sections 3.3, 3.13, 6.2, 6.7, 6.10 and 6.16
- MIL-DTL-5541F, Chemical Conversion Coatings on Aluminum and Aluminum Alloys, US Department of Defense, 2006; sections 1.2, 3.3, 3.4 and 6.1
- ASME Y14.5-2018, Dimensioning and Tolerancing, ASME, 2018 (reaffirmed 2024)