Guide
How do you specify powder coat masking for threads, bores and ground points on a drawing?
Powder coat masking is the practice of covering selected features of a part with heat-resistant plugs, caps or tape before it is pretreated, sprayed and cured, so that those features come out of the oven as bare metal while the rest of the part is coated. On a drawing it is a requirement like any other: it has to name each masked feature, show the boundary, and say how the result is accepted, or it will be neither priced nor done.
By Saola Manufacturing engineering team · Published · Updated
This guide is for design and quality engineers who release powder-coated steel, aluminum or zinc parts, and for buyers who keep receiving housings whose screws will not start or whose bearings will not press in. It deals only with powder: a thick, baked polymer film that behaves very differently in a hole than a thin anodic or plated layer does. Masking for anodizing has its own guide, linked at the end, and so does choosing between finishes. What follows is the arithmetic that shows why coated threads and bores fail, the three ways to keep a feature functional, how to draw and note the masked area, what grounding studs and hang points need, and what the RFQ should say.
On this page
Why do powder-coated threads and bores fail at assembly?
Because a powder film is thick compared with a thread or fit tolerance, and the geometry of a 60° thread multiplies it. Even a thin powder film closes a tapped hole by more than its whole class 2B tolerance.
Powder coating has no single governing thickness standard; the film a given powder needs comes from its data sheet, and typical cured films on industrial parts run from about 0.0015 to 0.004 in (38 to 102 µm). That is several times the thickness of typical zinc plating or Type II anodize. Per MIL-A-8625F paragraph 6.10.2, the pitch diameter of a 60° thread increases by four times the growth of each coated surface (MIL-A-8625F). That specification is written for anodizing, where growth is only about half the film, but the rule itself is thread geometry; a powder film sits entirely on top of the metal, so its growth is its full thickness. A bore loses twice the film on its diameter.
Now set that against the tolerances. Per the Unified thread tables published as ASME B1.1, the pitch diameter of a 1/4-20 UNC-2B hole runs from 0.2175 to 0.2224 in, a tolerance of 0.0049 in (NBS Handbook H28). Per ISO 286-1, an H7 hole at 10 mm nominal has a tolerance band of only 0.015 mm (ISO 286-1). The table shows what a powder film does to both.
| Cured film per surface | Thread pitch diameter closes by (in) | Bore diameter closes by (in) |
|---|---|---|
| 0.0015 in (38 µm) | 0.0060 | 0.0030 |
| 0.0020 in (51 µm) | 0.0080 | 0.0040 |
| 0.0030 in (76 µm) | 0.0120 | 0.0060 |
| 0.0040 in (102 µm) | 0.0160 | 0.0080 |
The real picture is worse than the table, not better. Electrostatic powder does not lay down evenly inside a hole: charged particles are pulled to the nearest grounded edge, so the crests and the hole entry build up while the thread root and the bottom of a blind hole may get little or nothing. That shielding effect is usually called the Faraday cage effect. The result is a tapped hole that is neither cleanly coated nor cleanly bare, with a lump of cured film at the first thread that stops the screw. No thread class and no oversize tap can absorb that, which is why the practical answer for powder is almost always to keep the coating out of the feature in the first place.
Which features need powder coat masking, and which can stay coated?
Mask any feature whose function depends on its size, its fit or its bare metal: tapped holes, studs, fit bores, locating pins and holes, mating and sealing faces, and electrical bonding points. Leave coated everything that only needs to look right and resist corrosion.
A useful test is to ask what the next part in the assembly touches. If a fastener threads into it, a bearing or bushing presses into it, a dowel locates in it, a gasket or O-ring seals on it, or a wire lug or EMI gasket must make electrical contact with it, coating will break that function and the feature belongs on the masking list. Clearance holes for bolts are the common exception: a generous clearance hole can usually take the film, though the bolt head will then clamp onto powder, which can creep and lose preload over time. If clamp load matters, mask the spot face under the head as well.
Think about the edges too. A plug in a tapped hole leaves the countersink or chamfer either coated or bare depending on the plug shape, and a coated chamfer can still lift a flake into the first thread. Decide which you want and say so. Equally, a masked bearing bore usually needs the face it shoulders against kept bare, or the bearing will sit on a layer of soft polymer instead of metal. The guide on press-fit tolerances for dowel pins and bearings shows how little room a fit bore has to give away.
Finally, do not forget what the masked area becomes. Bare steel under a removed mask will rust in transit and storage. Either the part is plated or otherwise protected under the powder, or the drawing names a rust preventive for the masked features after coating. Bare aluminum is more forgiving, but if the drawing wants a conversion coating on the masked areas, say so explicitly.
Should you mask, machine after coating, or chase threads after coating?
Masking is the default for threads and fit bores. Machining after coating suits a few precise features on an otherwise simple part, and chasing threads after coating is only acceptable for low-duty threads where a bare, scraped flank does no harm.
Masking keeps the feature exactly as machined. Tapped holes are usually closed with high-temperature silicone plugs or with a sacrificial screw run in before coating, studs with silicone caps, bores with plugs or caps, and flat faces with high-temperature polyester or polyimide tape or a die-cut mask. The mask must survive pretreatment chemistry and the cure oven, and it should come out cleanly. When a cap or tape is pulled, the cured film can tear at the edge and leave a ragged lip, so experienced coaters cut the film at the boundary first. Masking costs labor per feature, every time, which is why it must be counted at quote stage.
Machining after coating means the hole is drilled, tapped or reamed through the cured coating, or the blank is coated before its final operations. It gives a perfect bare feature and avoids masks, but the cut can chip the coating at the hole edge, the fixture can mark coated faces, and the part must survive a second handling through the machine shop with a finished cosmetic surface. It works best when there are only a few such features and the part is easy to fixture.
Chasing means running a tap or die through the coated thread to clear it. It is quick, but it scrapes the flanks bare in some places and not in others, can push flakes of film into a blind hole, and on steel leaves exposed metal without any protection. It is a rework, not a process, and a drawing should only permit it deliberately. The guide on threads too tight after anodizing or plating covers the thin-film case, where cutting oversize before finishing is a real option; with powder it rarely is.
How do you show masked areas on the drawing?
Tie a flag note to every masked feature and, for areas on a face, show the limits with a chain line and dimensions, the convention ASME Y14.5 uses for a requirement that applies only to a limited area. Then add one general note that says what masked means and how the boundary is accepted.
The general finish note names the coating; the masking information should never live only in that note, because a list of words like “mask all threads” leaves the coater to decide what counts. A flag note on each feature removes the guesswork and lets the first-article report check each one. For a face, use the ASME Y14.5 limited-area chain line with dimensions locating its ends (ASME Y14.5); the guide on masking for anodizing walks through that convention and the general masking note, which carry over to powder unchanged. Give the boundary a tolerance that matches how it will be made: plugs and caps follow the feature edge, while hand-cut tape needs a looser band, and a tight boundary on a face nobody sees only adds cost.
Also say whether dimensions apply before or after coating. For masked features it is simple, because they stay as machined, but for coated features a mating dimension may need a note such as “dimension applies after coating”. The guide on surface finish before or after coating covers the same question for roughness callouts.
| Feature | Example flag note | What it settles |
|---|---|---|
| Tapped hole | FLAG 1: MASK THREAD FULL DEPTH INCL. CHAMFER. NO POWDER COAT. GAUGE 1/4-20 UNC-2B AFTER COATING. | Depth, chamfer, and that the gauge check happens after the oven |
| Fit bore | FLAG 2: MASK BORE AND SHOULDER FACE. BARE METAL. BORE SIZE APPLIES AFTER COATING. | Bearing seats on metal; inspection point |
| Ground point | FLAG 3: MASK SPOT FACE Ø.50 AROUND GROUND STUD HOLE BOTH SIDES. BARE METAL FOR ELECTRICAL BONDING. | Contact area on both faces of the joint |
| Sealing face | FLAG 4: MASK AREA WITHIN CHAIN LINE. COATING EDGE WITHIN .03 OF DIMENSIONED LIMIT. NO LIFTED EDGE. | Boundary location, tolerance and edge quality |
| Hang point | FLAG 5: RACK CONTACT PERMITTED IN THIS HOLE ONLY. CONTACT MARK ACCEPTABLE. | Where the coater may hook the part |
| Masked steel | MASKED AREAS: APPLY RUST PREVENTIVE AFTER MASK REMOVAL. | Protection of bare metal after coating |
What do grounding points and hang points need?
A grounding point on the finished product must be bare metal at every contact surface, and a hang point is where the coater grounds the part during spraying. Both should be chosen by the designer, not left to the paint line.
Powder is a good electrical insulator once cured. Where a powder-coated enclosure, bracket or frame must be bonded to an equipment ground, NFPA 70, the US National Electrical Code that governs installations, requires in its clean-surfaces rule (section 250.12) that nonconductive coatings such as paint and enamel be removed from threads and contact surfaces to ensure electrical continuity, unless fittings designed to make removal unnecessary are used. In drawing terms, that means masking the spot face around the ground stud on both sides of the panel, and masking the threads of a welded or clinched ground stud, rather than relying on a serrated washer to bite through a film of unknown thickness. If the part will be bonded through a mating flange, the flange faces that touch need the same treatment.
Hang points are the other side of the same physics. Electrostatic spraying works because the part is grounded through its hook or rack, so the contact point stays uncoated, and as hooks build up cured powder the grounding gets worse and transfer efficiency drops. A coater will choose a hole to hang the part from unless the drawing says otherwise, and that hole may be the one feature you needed coated, or a cosmetic edge that ends up with a bare notch. Naming one permitted hang point, ideally a hole that is masked anyway, settles both problems.
How do you check masked features before the parts ship?
Check every masked thread and bore with gauges after coating and cure, not before, and judge the coating boundary against an agreed sample. Measure film thickness away from the mask edge so the edge build-up does not distort the reading.
The checks that matter happen after the oven, because that is when a missed plug or a flake of film shows up. Thread GO and NO-GO plug gauges on tapped holes, and plug or pin gauges on bores, will catch both an unmasked feature and a mask that left debris behind; the guide on go/no-go thread gauges explains how to read the result. Bonding points can be checked for continuity with a simple meter across the masked area.
Film thickness on steel and aluminum is measured with magnetic or eddy-current gauges, and per ASTM D7091 readings near edges are less reliable, so the measurement areas should be on flat coated faces (ASTM D7091). The quality of the mask line, meaning edge lift, ragged film and overspray into the bare area, is a cosmetic attribute with no numeric scale in common use; a boundary sample signed off at first article is the most reliable way to judge it. Saola’s quality page states that parts are finished to the cosmetic standard the buyer supplies, whether a boundary sample, a written specification or marked-up photos, and that if none is supplied it asks for one before production.
What should the RFQ and purchase order say about masking?
List the masked features by flag number with a count per part, say which masking approach is acceptable for each, and state the acceptance checks. Masking is labor per feature, so a quote can only include it if the RFQ shows it.
Saola offers powder coating through partner plants it manages, as part of its surface finishing menu, and that page makes the same point this guide does: threads, ground contacts and press-fit bores usually need to stay free of coating, and buyers are asked to call them out explicitly so they are masked correctly the first time. The same page notes that finishing cost is driven by racking, masking and changeovers more than by the coating itself. Its menu covers eight finishes and no others; a conversion coating for bare aluminum areas, for example, is not on it (the blue trivalent chromate it lists is a topcoat for zinc plating), so raise any such requirement before the quote.
A complete masking package in the RFQ has five parts. First, the drawing with flag notes on every masked feature and the chain-line limits for faces. Second, a short table of masked features: flag number, feature, count per part, method allowed (mask only, or mask or machine after coating). Third, the hang point you will accept. Fourth, how the bare areas are protected after coating. Fifth, the checks at first article and per lot, usually gauge checks of masked threads and bores plus a boundary sample. The guide on preparing a machining RFQ covers the rest of the package, and the comparison of anodizing, powder coating and zinc plating helps if a thinner finish would remove the need to mask at all.
Frequently asked
Can I skip masking and just re-tap the holes after powder coating?
For a few low-duty holes, sometimes, but treat it as a decision you write on the drawing, not a shortcut the supplier takes. Re-tapping leaves bare, scraped flanks that rust on steel, can push film into a blind hole, and can cut a crossed or oversize thread if the tap does not follow the original. Fit bores cannot be rescued this way at all without reaming, which changes the size the drawing controlled.
Powder hardly gets into deep holes anyway. Do they still need masking?
Yes. The shielding that keeps powder out of the bottom of a hole also concentrates it at the entry, so the first threads, the chamfer and the hole edge often carry the heaviest build of all. That partial coating is the worst of both outcomes: it blocks the fastener and still does not protect the hole. A plug costs less than a rejected lot.
How tight should the masking boundary on a face be?
Only as tight as the function needs. A sealing land or a bonding pad needs its edge controlled; a cosmetic transition on a hidden face does not. A boundary that follows a feature edge, such as a spot face or a step, is easier to hold than a line in the middle of a flat face, so where you can, design a small step or groove for the mask to follow.
Is masking for powder coating the same as masking for anodizing?
The drawing logic is the same, but the reasons and the materials differ. Anodizing grows a thin, predictable layer partly into the metal, so some threads and bores can be sized to take it; powder is far thicker and uneven in holes, so masking is nearly always the answer. Powder masks must survive the cure oven, and the cured film can tear at the boundary. The guide on masking for anodizing covers the anodizing side.
Does a die casting change how masking is specified?
The callouts are the same, but castings add two things to check. Machined faces and tapped holes on a casting can expose porosity, which may outgas under a nearby coating, and plugs pressed into a cast hole must seal against a surface that is not always perfectly round. See powder coating die cast aluminum without outgassing and powder coating zinc die castings.
Sources
- MIL-A-8625F, Anodic Coatings for Aluminum and Aluminum Alloys, US Department of Defense, 1993 (Amendment 1, 2003)
- NBS Handbook H28, Screw-Thread Standards for Federal Services, Part I, National Bureau of Standards (now NIST), 1969
- ISO 286-1, ISO code system for tolerances on linear sizes, Part 1, International Organization for Standardization, 2010
- ASME Y14.5, Dimensioning and Tolerancing, American Society of Mechanical Engineers, 2018
- ASTM D7091, Nondestructive Measurement of Dry Film Thickness of Nonmagnetic Coatings, ASTM International, 2022