HOW TO CHOOSE A
COATING VENDOR.

Why coating is hard to buy well

With most machining and fabrication you can measure the result. With coating, the two attributes that determine whether the part performs — adhesion and degree of cure — are invisible on a finished part. A properly cured coating and an under-cured one look identical coming out of the oven. The difference appears months later in the field, usually on the customer's assembly line rather than yours.

That asymmetry is why vendor selection matters more here than in most purchased processes. You are buying process control you cannot verify by inspection.

1. Will you run a sample batch first?

Any applicator confident in their process will coat a small batch of your actual parts so you can test them before committing. A shop that resists this, or wants full production pricing to run a handful of parts, is telling you something. Sample runs also surface masking and fixturing problems while they are cheap to solve.

2. How do you verify cure?

This is the highest-yield question on the list, and the answers separate shops immediately. What you want to hear is that they measure part temperature, not oven set point — using thermocouples on representative parts or documented oven profiling, with heavier parts profiled separately because mass changes the ramp. If the answer is a set point and a timer, cure is being assumed rather than controlled.

3. How do you measure and control film thickness?

Thickness drives both performance and fit. Too thin and the coating wears through early; too thick and coated features fall out of tolerance, threads bind, and press fits stop pressing. Ask what instrument they use, how often they check, and what they do with parts outside the range. On critical dimensions, ask whether they can hold a specified thickness band rather than a nominal target.

4. How do you test adhesion?

Adhesion is the failure mode that generates warranty claims. Standard cross-hatch tape testing to ASTM D3359 on a coupon processed alongside your parts is a reasonable baseline. The follow-up question is what happens to a batch that fails — and whether they keep the coupons.

5. What is your masking capability?

Most coating problems that reach the assembly line are masking problems. Overspray onto a bearing journal, a sealing face, or a thread flank means rework or scrap. Ask how they mask, whether they build custom fixtures and hard tooling for repeat parts, and how they handle blind holes and internal features. A shop with real hard masking capability is a different proposition from one working with tape and plugs.

6. What documentation ships with the parts?

For anything qualified to a specification, a certificate of conformance should be routine rather than an upcharge. Ask what the certificate actually states, whether lot traceability is maintained, and how long records are retained. If you are working to MIL-PRF-46010, MIL-L-46147, AS5272, or a customer-specific standard, confirm they have processed to it before rather than that they believe they could.

7. What is your real lead time — and your real capacity?

Ask two versions of this question: what is the lead time on the quoted batch, and what happens when you need triple that quantity in the same window. The second answer tells you whether the shop can grow with the programme or becomes a bottleneck at the point where volume finally justifies the tooling.

Red flags

• Pricing well below the field with no process explanation. Coating cost is dominated by labour, masking, and oven time. A large discount usually means one of those is being skipped, and the one most often skipped is cure time.
• No willingness to discuss specific formulations. A shop that answers every requirement with the same product is selling what it stocks, not what your part needs.
• Vague answers about supplier relationships. Ask which coating lines they work with directly. Direct supplier relationships mean access to technical support and current formulations.
• Minimum order quantities that prevent prototyping. If you cannot get ten parts coated, you cannot iterate a design.
• No sample programme. Covered above, and it remains the fastest single filter.
• You cannot reach a technical person. When something goes wrong at two in the afternoon on a Friday, you need an engineer on the phone, not a ticket number.

What good looks like

A capable applicator will ask you more questions than you ask them. Substrate alloy, service temperature, contact pressure, what the coated surface touches, which features are dimension-critical, what the part failed at last time. A shop that quotes from a part number alone has not engaged with the application, and the coating recommendation that follows is a guess dressed as a specification.

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