A moulded elastomer component is often only half the specification. The other half sits on the surface: a coating, a primer, a release film or a functional treatment that changes what the part does at the interface where it meets everything else. Well-chosen surface treatment turns a marginal seal into a reliable one, or a difficult assembly into a routine one. Poorly-chosen — or omitted where it should have been specified — it becomes the first line of the failure report.
Here is a working tour of the surface treatments we most often apply, and where each earns its place on the drawing.
Low-friction coatings
The commonest complaint on incoming enquiries: "the seal binds on installation" or "the shaft squeals on start-up". Both are surface-friction problems, and both are commonly solved by a low-friction coating applied to the elastomer.
**PTFE coatings** — sprayed, cured, and often applied in tumbling drums for volume production — reduce the coefficient of friction of an elastomer surface by an order of magnitude. On O-rings destined for automated assembly, a PTFE coating is often the difference between a seal that installs cleanly with a robot and a seal that jams the machine.
**PFPE (perfluoropolyether) treatments** are a lighter alternative — a film of low-friction lubricant chemically bonded to the surface rather than a physical coating. They reduce insertion friction and stick-slip on dynamic seals without changing the dimensional envelope of the part.
**Silicone-based release films** are the cheapest option, appropriate where the requirement is a one-time installation and long-term friction is not a concern.
The economic case for a low-friction coating is usually straightforward: on any automated line, the coating pays for itself in reduced installation-damage rejects within weeks. On dynamic seals, the pay-off is longer service life and reduced start-up torque.
Bonded primers
For rubber-to-metal or rubber-to-plastic bonded assemblies, the primer-and-adhesive system is a surface treatment applied to the substrate, not the elastomer, but it belongs in the same conversation. The full detail is in a separate post on bonded assemblies; the short version is that primer choice determines whether the bond survives the service environment, and it should be specified against the elastomer family, the substrate material and the service conditions.
Environmental coatings on the metal insert
Bonded assemblies often specify a surface treatment on the metal insert as well as the primer that follows it. A **zinc plate** or **zinc-nickel plate** on a steel insert prevents in-service corrosion at any point where the metal is exposed after bonding. A **cadmium plate** is still specified on some aerospace applications for its performance in salt-spray, though its use is falling. A **phosphate conversion** provides a lightly-textured corrosion-resistant surface that also improves primer adhesion.
The interaction between the substrate coating and the bond primer is not always intuitive. A part specified with a zinc-nickel plate and then bonded with a primer intended for bare steel will not perform the same way as either specification would in isolation. The bond system should be selected against the coated substrate, not the bare substrate.
Moisture barriers on hygroscopic compounds
Certain elastomer compounds — some polyurethanes in particular — are hygroscopic and lose mechanical properties as they take up ambient moisture. Where the component will sit in a humid environment for long periods before use, or where dimensional stability is critical, a **moisture-barrier coating** applied to the elastomer keeps the compound at its as-moulded state.
Silicone and fluoro-based coatings both perform this role well. The choice depends on the elastomer chemistry and the service temperature.
Chemical-resistance coatings
Where the elastomer's own chemistry is not quite enough for the fluid environment — the compound family is otherwise correct, but the fluid is at the edge of what the compound will tolerate — a thin chemical-resistant coating on the elastomer can extend its life significantly. Fluoropolymer coatings applied to nitrile, EPDM or silicone parts are the common approach. The coating provides the chemical barrier; the elastomer underneath provides the mechanical properties.
This approach only works where the coating adheres well, does not itself degrade in service, and does not affect the seal geometry beyond acceptable limits. It is not a substitute for correct compound selection, but it can extend the working envelope of a compound that is otherwise the right choice.
Colour, identification and marking
Occasionally the "coating" is a marker, not a functional layer. Colour-coded seals are common on multi-variant assemblies where the wrong seal in the wrong groove is a safety issue: red for the high-pressure port, blue for the low-pressure port, and so on. Coloured compounds are used where the colour must be integral; painted or coated markings are used where the base compound is fixed by other requirements.
Laser marking of a batch code onto the elastomer is now common on aerospace parts. The mark must be legible, permanent, and not compromise the surface finish in the sealing area.
When a coating is the wrong answer
A coating cannot rescue a fundamentally wrong compound choice. If the base elastomer is chemically incompatible with the service fluid, a coating that lasts a hundred hours will not save it. If the compound cannot cope with the temperature, a coating that fails at 200 °C will not extend the service life beyond 200 °C.
The correct sequence is: choose the compound for the fundamentals, then choose the surface treatment for the specifics of installation, friction, life extension or identification.
What to send us
If a drawing calls for a specific coating specification — a named PTFE grade, an aerospace-listed primer, a defence-listed plating — we source and apply to that specification. If the requirement is functional — "must slide into aluminium bore without scoring", or "must not shed particles in a clean environment" — we recommend a coating and specify it on the drawing revision.
Either way, the coating goes on the FAIR: the specification, the process, the batch and the certificate travel with the part. A coated component with a certificate that does not identify the coating is a component that cannot be audited, and in regulated sectors it is a component that cannot be delivered.
