Coating masking for sheet metal: what to mask and why

A powder-coated enclosure left the line with 75 µm on the face, a clean cross-hatch and a protective bonding resistance of 1.8 MΩ at the earth stud. The coating was correct. The part was unusable, because the stud that ties the panel to the building earth was buried under the same film as the outside.

Coating masking is the set of decisions about what stays bare. Get it wrong in one direction and the part fails electrically, mechanically or at assembly. Get it wrong in the other direction and every feature is masked, the coating misses the areas that need protection, and handling time doubles.

The rule is short: mask anything that must conduct, mate, thread, seal or be welded after coating. Everything else gets coated. The rest of this article is the numbers behind that rule and the way to put it on the drawing.

What has to stay uncoated?

Five categories, and only five: electrical bonding points, threads, precision mating faces, moving interfaces, and areas that will be welded after coating.

Each has a different tolerance for film. A bonding pad tolerates zero film at the contact face because the connection is measured in tenths of an ohm. A mating face tolerates maybe 20 µm before the stack-up moves. A thread tolerates almost nothing at M8 and below, and a decorative face tolerates the full 60–80 µm.

The film is the same in every case. Powder builds 60–80 µm per coat on a vertical face, and slightly less on edges and in recesses. That is 0.06–0.08 mm per surface, or 0.12–0.16 mm across a dimension where two coated faces meet. On a tolerance stack measured in tenths of a millimetre, that is the whole allowance.

What 80 µm does to a thread

An M8×1.25 internal thread has a basic minor diameter of 6.647 mm. Powder wraps both flanks, so the effective minor diameter falls by twice the film: 6.647 − 0.16 = 6.487 mm. That is below the basic minimum, which means a standard M8 bolt cannot enter far enough to engage.

At 40 µm per flank the number is 6.567 mm, still under. The thread only becomes usable if the film is under about 20 µm, which no powder process holds reliably inside a blind hole. So M8 and below get plugged. M10 and above have enough allowance to coat and chase with a tap, but chasing is an operation, and an unchased thread is a rework ticket.

What 80 µm does to an earth bond

IEC 61439-1 requires the protective circuit between exposed conductive parts of an assembly to have a resistance of 0.1 Ω or less. A cured polyester powder film is a dielectric; measured across 75 µm it reads in the megohm range, not the milliohm range.

A star washer does not solve this in a controlled way. It bites through the film at a few points, the contact area is tiny, and the resistance depends on how hard the nut was driven. Masking a 20 mm pad at one stud per panel gives a bare, conversion-coated surface, a defined contact area and a test that repeats. That is the difference between a part that passes the routine test and a part that passes once.

The masked pad is not bare steel in the finished part. It carries the conversion coating from pretreatment, typically 1–3 g/m² of zinc phosphate on CRCA, and that coating is thin enough to be displaced by a star washer under torque. What it must not carry is powder, because 60–80 µm of cured polyester cannot be displaced by a washer at any sensible torque. The test is the same either way: measure between the stud and the enclosure body, and record the value on the first article.

Coating masking schedule by feature

Put this on the drawing as a table, not as a note. The shop needs to know the feature, the mask size, and whether the area is allowed to carry a conversion coating.

The Ganesh column reflects what the line can hold. A 20 mm pad is a standard die-cut disc. A 50 mm weld band is masked with tape and inspected before the booth. What we will not do is promise a masked area that stays bare through a seven-tank line without a plug that can survive it, because the chemistry finds every gap.

How much masking costs

Masking is manual. A silicone plug takes 10–20 seconds to fit and the same to remove; a taped datum face takes 30–40 seconds each way. On an enclosure with 30 masked features, that is 10–20 minutes of handling before and after the booth, on every part, regardless of order size.

The cost does not scale down with volume the way powder consumption does. It scales with feature count. That is why the first design review question is not "can you mask this?" but "how many masks does the part need?"

Racking compounds it. A masked part cannot be hung by the masked face, so the rack either touches the coated surface or the design needs a dedicated hanger hole. Small parts masked on one face can share a rack; a 1.5 m panel cannot. Over a 50-part batch, the difference between a one-face mask and a two-face mask is a second handling cycle for the whole batch.

Three design moves cut the count without losing function:

Consolidate bonding points. One 20 mm bare pad with a stud, bonded to the other panels by a braided strap, replaces four masked studs. The electrical result is the same and the mask count drops by three.

Move masked features to one face. A part that needs masking on two opposite faces is handled twice and racked twice. Grouping the earth stud, the cable entry and the thread inserts on the bottom flange reduces handling and makes the mask easy to inspect.

Use post-coating hardware. Rivet nuts, clinch nuts and standoffs can be fitted after cure where the joint allows it. A blind rivet nut in a coated hole is a defined operation; a masked thread is a mask that can fall out.

The failure a missing plug causes

A plug that drops out in the booth leaves a coated thread. The part goes to assembly, the bolt starts two turns and stops, and the fitter either forces it or sends the panel back. Forcing it strips the coating and leaves a chip ring that fails the finish inspection. The part is now scrap or rework, and the cost is the whole panel, not the plug.

A plug that stays in but seats badly leaves a ring of thin film at its edge. That ring is where corrosion starts, because the film thickness steps from 75 µm to zero over a millimetre. Blasting or a chamfer at the mask boundary helps, but the real control is a plug that fits the hole and a first-off inspection that checks the boundary.

Masking and the corrosion class

ISO 12944-2 classifies the corrosivity of the environment, from C1 indoors to C5 at sea or in chemical plant. The class decides the coating system, not the masking, but it changes the cost of a missed mask. A missed earth pad on a C2 indoor panel is an electrical fault. The same miss on a C4 coastal panel is a corrosion site, because the bare pad is a bare steel window in an otherwise protected surface.

If the part is C4 or worse, every masked area needs a plan for what happens after coating: a stainless stud and a sealed washer, a zinc-rich touch-up at the boundary, or a gasket that keeps water off the bare face. A bare pad in a wet enclosure is a design decision, not a masking detail.

Writing the schedule on the drawing

1. A masking table with feature, mask diameter or area, and the operation where the mask is fitted.
2. Hatched or shaded areas on the view, with the note "NO COATING" and a leader to the feature.
3. Thread callouts with the class after coating, for example M10-6H after chase.
4. Earth stud location and the test requirement: 0.1 Ω maximum, measured between the stud and the enclosure body.
5. The coating class and film build per ISO 2808, including where the measurement points are.
6. A note on the mask boundary: "no film step greater than 20 µm at masked edge", if the part is C4 or the gasket is critical.
7. Whether the masked area may carry a conversion coating. On an earth pad, a thin conversion coating is acceptable under a star washer; on a mating face it is not.

The paint shop capability page sets out the seven-tank line, the 18 ft × 14 ft powder booth and the wet paint booth. Masking is quoted as its own operation there, because it is labour, not chemistry. For the preparation side, degreasing before powder coating explains what the tanks do and why a masked surface behaves differently from a blasted one.

The design and CAD hub indexes the rest of the drawing-note guides. Powder coating laser-cut edges covers the edge condition that meets a mask boundary, and box bending for electrical enclosures covers the enclosure geometry that decides where the masks land.

What Ganesh actually runs

Coated enclosures go through seven-tank pretreatment, then the 18 ft × 14 ft electrostatic powder booth or the wet paint booth, then the curing oven. Film is checked with a DFT gauge and adhesion with a cross-hatch test. Masking is done by hand between dry-off and the booth, with silicone plugs and die-cut tape, and the mask schedule from the drawing is transferred to a shop card for each batch.

Parts for Mahindra, Godrej & Boyce and TKE run through this line, including electrical enclosures where the bonding test is part of the first article inspection. The data centre rack product page shows the kind of panel where earth continuity, cable entry and thread masking all meet on one part.

Where the drawing has no masking callout, we quote the part with the earth stud and threads masked as a default and flag it at quotation. That default is a conversation, not a specification: a customer who intends to chase threads after coating should say so, because the two routes price differently.

When masking is the wrong answer

Masking is a manual operation defending a coating that should not be there. Four cases where the design should change instead.

Large areas. Masking a 200 × 300 mm window for a nameplate or a label is slower and less reliable than coating the whole face and fitting the plate over it. If the area is bigger than about 50 mm across, ask whether it needs to be bare at all.

Threads that will be chased anyway. If the assembly uses a tap or a thread-forming screw, a coated thread may be acceptable. Specify the class after coating and let the shop chase; do not mask and then chase, which is two operations for one result.

Selective coating by hand. We do not laser-ablate coating or brush-plate a masked area. If a part needs a conductive path that cannot be masked, the answer is a different coating system or a mechanical bond, not a touch-up in the aisle.

C5 or immersion service. A bare pad in a C5 enclosure will corrode unless it is protected by a sealed connection. Masking solves the electrical test and creates a corrosion site. In that case the design needs a stainless stud with a sealed washer and a documented torque, or a coated surface with an external bonding strap.

Send the drawing with the earth studs, thread callouts and mating faces identified to request a quote. The masking schedule is reviewed with the coating route, so the handling and the film specification are priced together rather than discovered at first article.