Powder Coating Masking: What to Mask and How to Call It Out

Why masking matters on a production part

Powder coating is a thick film. Cardinal’s T009 TGIC polyester data sheet lists a 1.5 to 3.0 mil test film, and its C909 superdurable sheet lists 2.5 to 4.0 mils. That is per surface. On a bore or a shaft, the change in diameter is twice the film thickness, so a 3 mil film takes about 0.006 in. off a hole.

On a cosmetic panel that build is a benefit. On a press fit, a tapped hole or a gasket face, it is a defect. Masking keeps powder off the surfaces that have to stay bare so the part goes straight from our dock into your assembly cell, without anyone chasing threads or scraping a face with a razor blade.

Masking is also the step most likely to be done inconsistently if it is not written down. The rest of this guide is about getting it right on the drawing the first time.

What usually needs masking

Threads and tapped holes

Internal threads are the most common callout. Powder builds on the thread flanks and at the entry of the hole, and even a thin film can make a fastener bind, gall or cross-thread. Blind holes are a special case. Products Finishing explains that the bottoms of blind holes act like inside corners and resist coverage (the Faraday cage effect), so powder tends to collect at the mouth of the hole rather than inside it. That mouth is exactly where the fastener starts.

External threads on studs and weld-on bolts need the same attention. Press-in hardware such as nuts, studs and standoffs installed before coating should be called out too, or it will come back coated along with the rest of the part.

Bores, bearing seats and press fits

Any diameter with a tight tolerance is a masking candidate: bearing bores, bushing seats, dowel holes, shaft journals and slip fits. If you have not budgeted for film build in the tolerance stack, assume the feature needs to be masked.

Mating, sealing and datum surfaces

Faces that seat a gasket or O-ring, bolt flat against another part, or slide in a track usually stay bare. So do datum surfaces your receiving inspection measures from. If the coating is on the datum, you are measuring paint, not metal.

Ground and electrical contact points

A cured powder film is a poor electrical conductor. Wherever an assembly depends on metal-to-metal contact for grounding, bonding or EMI shielding, that contact area has to be bare. Typical examples are ground stud locations on an enclosure, bonding points on a chassis rail, and the faces where a door or panel must make continuous contact with the frame.

Call these out with a size and a side. “Bare metal, 0.75 in. diameter minimum, both faces” is a lot easier to execute consistently than “keep ground clean.”

Hang points and rack contact

Every part we coat is hung on a hook or rack, and that contact point has to stay clean to carry the electrical ground the process depends on. Products Finishing stresses clean contact points as a basic requirement for good transfer efficiency and coverage. The result is a small uncoated or marked spot where the part touched the rack. If there is a place on the part where that is acceptable (an inside flange, a hidden tab, an existing hole), note it on the drawing. If there is a place where it is not, note that too. Our guide to designing parts for powder coating covers where to put hang holes.

Masking methods and when each one fits

High-temperature tape

Polyester-film tape with a silicone adhesive is the standard choice for powder coating. 3M 8992 and Caplugs PC21 are both rated to 400°F, which covers a typical cure of around 400°F at the part. Caplugs states its PC21 adhesive will not leave an adhesive residue. Heavier 5 mil polyester versions are made for larger surfaces where tear strength matters.

Polyester backings start to shrink as bake temperatures climb, so for a hotter cure or a long outgassing pre-bake, polyimide and glass-cloth tapes rated to about 500°F are the safer choice. Tape is best for straight lines and flat faces. Hand-cutting tape around curves and holes is slow, which is where die-cut shapes earn their keep.

Silicone plugs and caps

Plugs go into holes, caps go over studs and shafts. Silicone versions are rated far above a powder cure (Caplugs lists 600°F for its Ultrabake line) and are generally reusable. Other materials are cheaper but come with limits: Caplugs rates EPDM caps to 350°F, which is below a standard 400°F cure, and its vinyl material is intended for one-time use. If you supply your own masking, check the rating before it goes into our oven.

For threaded holes, a tapered or stepped plug covers a range of sizes and seals the thread entry. Caplugs also makes silicone grounding caps for tabs, and washer caps that plug a hole or cover a stud while leaving a bare grounding ring around it.

Custom masks

When a part number repeats and the masking is complex (a cluster of bores, an irregular sealing face, a pattern of studs), a custom die-cut or molded mask can replace dozens of hand operations with one. Caplugs and similar suppliers design molded silicone and EPDM masks to a part drawing. The trade-off is tooling cost and lead time up front. On a run of a few dozen parts it rarely pays. On a blanket order for thousands of the same part, it often does. We can talk through whether it makes sense for your program, and our manufacturer and OEM page covers how we handle repeat part numbers.

Silicone and downstream processes

Silicone is the default adhesive and plug material because it survives the oven. If the masked area will later be painted, bonded or sealed, silicone transfer can be a concern. Non-silicone high-temperature tapes exist for this situation. Put the downstream process on the PO and we will choose the masking to suit.

Removing the mask

After cure, the film often bridges over the edge of tape or a cap. Pulling the mask hard can lift coating off the part along that line, so the film at the edge is cut where needed and the mask comes away cleanly. The goal is a crisp boundary with no loose coating left at the edge, because a loose edge is where peeling starts later.

How masking drives cost

Coaters build a piece price from how many good parts they can get through the line per hour. Products Finishing calls racking density critical and says racking and throughput should be settled before a job is quoted. Masking cuts into throughput in two ways:

1. Hand labor on every part. Each plug, cap and strip of tape is placed and removed by hand. A bracket with no masking can be racked and sprayed in seconds. A housing with twenty tapped holes and a masked face takes minutes. This is the main reason our daily output ranges from about 500 to 2,000+ parts depending on part size and masking.
2. Consumables. Tape is single use. Reusable silicone plugs and caps spread their cost across runs, and custom masks spread theirs across a whole program.

Masking also adds risk. Every masked edge is a place where a callout can be misread or a plug can fall out in the booth. Clear callouts reduce rework for everyone.

Ways to keep masking cost down

• Mask only what is functional. Old drawings often carry masking notes that no longer serve a purpose. Ask whether each one is really needed.
• Allow alternatives where they work. If threads can be chased or bores reamed after coating, say so, and we can quote both ways.
• Install hardware after coating when the design allows. Press-in hardware added after coating needs no masking at all.
• Open up clearance holes enough to tolerate film build, instead of masking them.
• Standardize hole sizes so one plug size covers most of the part.

More cost levers are in our guide to powder coating cost factors.

How to call out masking on the drawing

A verbal request (“make sure you keep the threads clean”) rarely survives the trip from a phone call to a production traveler. Masking belongs on the drawing or the PO, where it gets quoted, inspected and repeated the same way every run. Good callouts have four parts: what to mask, where the boundary is, how much tolerance the boundary has, and what is allowed if masking is not practical.

Example notes

These are starting points. Adjust them to your part and drawing standards.

• MASK ALL TAPPED HOLES. NO COATING ON INTERNAL THREADS.
• MASK SURFACES IDENTIFIED WITH FLAG NOTE 4. MASK BOUNDARY ±1/16 IN.
• GROUND POINT: BARE METAL, 0.75 DIA MIN, BOTH FACES, 4 PLACES.
• BORE A AND BORE B: NO COATING. MASK FULL DEPTH.
• COATING IN CLEARANCE HOLES PERMITTED.
• THREADS MAY BE CHASED AFTER COATING IN LIEU OF MASKING.
• RACK CONTACT MARKS PERMITTED ON INSIDE FLANGE ONLY.

Make the zones easy to read

Hatch or shade the masked area on the view and tie it to a flag note. A separate masking sketch, or a marked-up PDF attached to the RFQ, works well when the main drawing is crowded. For assemblies, list every masked feature in one table so nothing is missed on the second sheet.

Put the boundary tolerance on paper

Tape lines are cut by hand. If a mask edge needs to land within a few thousandths, that is a machining requirement, not a masking one. Give the boundary a tolerance you can live with, and say which direction is acceptable: is a little coating on the bare zone a problem, or a little bare metal next to it?

Our coating callout checklist covers the rest of the callout, including powder, color, gloss, film thickness and prep.

Bringing it together

Masking is where design intent meets hand labor. A clear callout tells the coater exactly what to protect, keeps your piece price honest, and makes the hundredth batch match the first. We mask, plug and cap parts in-house as part of our prep and masking service, using tapes, plugs and caps rated for the cure. On production programs we follow your QC procedure for masked features, and a Certificate of Conformance is available on request. If you are not sure which features need masking, send the drawing with your request for quote and we will flag anything that looks like it will cost more than it needs to.