8 Questions Every Aerospace Buyer Should Answer Before Choosing a Metal Finishing Shop …In Connecticut or Anywhere
Does the shop hold the certifications your program actually requires?
Verify Nadcap accreditation for the specific finishing process …not just a logo on a website … and confirm supporting quality certifications such as ISO 9001.
Is the recommended finish appropriate for the part's actual job?
Passivation, black oxide, manganese phosphate, zinc phosphate, and dry film lubricants each solve different engineering problems. The correct answer depends on the application, not habit.
Can the supplier work directly from your engineering specifications?
Experienced finishers should understand MIL, AMS, and ASTM callouts, recognize obsolete specifications, and identify current equivalents before production begins.
How much does surface preparation influence the final result?
Cleaning, degreasing, blasting, and other preparation steps often determine whether a finish performs correctly over time. Excellent coatings begin with excellent preparation.
What testing and documentation will accompany every production lot?
Certificates of conformance, traceability, salt-spray testing, coating verification, and other documentation are often just as important as the finishing process itself.
Will the finish affect critical dimensions or mechanical performance?
Some coatings add measurable thickness while others do not. Tight-tolerance aerospace parts often require finishes selected specifically to preserve dimensional accuracy.
Can working with a nearby supplier reduce manufacturing risk?
Shorter transportation distances can reduce freight costs, speed turnaround, simplify communication, and lower the chances of damaging precision components during shipping.
Is the supplier helping you solve engineering problems … or simply processing parts?
The best finishing partners review drawings, ask technical questions, identify potential issues early, and help ensure the specified process matches the part's intended performance.
01.
Common Aerospace Purchasing Mistakes That Can Delay Production
Connecticut builds a lot of things that fly. From the engine and rotorcraft work anchored around Hartford and East Hartford to the dense network of machine shops in New Britain, Berlin, Plainville, Bristol, Southington, and Farmington, this is one of the most concentrated precision-manufacturing corridors in the country. If you make aerospace parts here, you are rarely more than a short drive from the people who machine, inspect, heat treat, and finish them.
That geography matters more than outsiders usually assume. Aerospace parts move. A component might travel from a machining cell to finishing and back for inspection before it ever reaches assembly. Every one of those trips carries freight cost, packaging, handling, and the small but real risk of damaging a precision part in transit. A finisher located near your operation can cut that friction down, and in some cases arrange pickup and delivery, which is why proximity earns a legitimate spot on the supplier scorecard.
JH Metal Finishing has been part of this local supply chain since 1953. Seven decades in one place, working with Connecticut aerospace and defense manufacturers, is not a marketing line. It is the reason the shop can look at a drawing and know what the part actually needs. This article is about how to make that same judgment yourself, and it starts with a single word that separates a real aerospace finisher from a general one.
02.
Start With Nadcap
If you take one thing away from this article, make it this. Before you evaluate price, turnaround, or anything else, confirm that the finisher holds Nadcap accreditation for the processes your part requires.
Nadcap is the National Aerospace and Defense Contractors Accreditation Program, administered by the Performance Review Institute. It is an industry-managed audit system that verifies a supplier can control the special processes aerospace depends on. A special process is one where you cannot fully confirm the result by later inspection. If a part is coated inconsistently or cleaned improperly, a dimensional check at the end will not catch it. The confidence has to come from how the process itself was run.
That is exactly the case with metal finishing. You can measure a part all day and still not see whether the coating was applied to spec. So the industry built a common audit for it, and today most aerospace primes and their major suppliers require Nadcap accreditation as a condition of doing business.
For a purchasing manager or a newer engineer, the practical takeaway is simple. Nadcap is often a market-access requirement, not just a quality badge. If your program flows down Nadcap requirements and your finisher does not hold accreditation for that process, the part cannot ship, no matter how good it looks.
Nadcap is also separate from AS9100 and ISO 9001. Your own shop may run an AS9100 quality system, but that does not cover a supplier's special processes. Nadcap does. JH Metal Finishing carries Nadcap accreditation alongside ISO 9001:2015 certification, which is the combination aerospace buyers should be looking for and, frankly, one that not every shop in the region can offer.
You can verify any supplier's status and scope yourself through the PRI online database. Check the scope, not just the logo. Accreditation is granted for specific processes, so a shop approved for one may not be approved for another.
03.
The Finishes, and How to Think About Them
Most of the real decision-making in aerospace finishing happens before you ever pick a vendor. It happens when you decide what the part actually needs. No process is universally best. The right one depends on the substrate, the tolerances, the corrosion and wear requirements, the operating environment, and what happens to the part downstream in assembly.
Here is how the common processes line up.
Passivation
Passivation applies to stainless steel. Machining and handling leave free iron on the surface, and that free iron is what rusts, even on a "stainless" part. Passivation chemically removes it and restores the natural chromium-rich layer that gives stainless its corrosion resistance. It does not add a coating or change dimensions in any meaningful way. If your part is stainless and needs to resist corrosion, this is usually the baseline, commonly called out to ASTM A967, with cleaning per ASTM A380.
Black Oxide
Black oxide is a thin black conversion coating for steel. Its strength is that it adds almost no dimensional buildup, so it works on parts with tight tolerances where you cannot afford the thickness of plating. It gives you a uniform black finish and modest corrosion resistance, especially when sealed with oil. Its limitation is exactly that. Black oxide is not a heavy-duty corrosion barrier on its own. It is often specified to MIL-DTL-13924 or AMS 2485.
Manganese Phosphate
Manganese phosphate is the finish to reach for when wear, friction, and break-in matter. The crystalline coating holds oil, which is why it shows up on sliding and mating parts, fasteners, and components that need to seat in without galling. It reduces the tendency of steel surfaces to seize against each other. Typical callouts are MIL-DTL-16232 Type M and AMS 2481.
Zinc Phosphate
Zinc phosphate leans more toward corrosion protection and surface preparation. It provides a good base for paint and other coatings and adds corrosion resistance in its own right, which is why it often appears as a pre-treatment step. Common specs are MIL-DTL-16232 Type Z and AMS 2480.
Dry Film Lubricants
Dry film lubricants solve a problem that oil and grease cannot. When a part needs controlled, low friction but wet lubricant would attract contamination, migrate, or fail at temperature, a bonded solid-film lubricant coats the surface and stays put. It reduces friction, wear, and galling in places where you simply cannot keep a wet lubricant. These are usually specified to MIL-PRF-46010 or MIL-PRF-46147.
Rust-Preventive Oils
Rust-preventive oils and supplementary treatments are the finishing touches that often ride on top of another process. A phosphate or black oxide coating is frequently sealed with oil to reach its rated corrosion performance. When your drawing calls for a supplementary oil or sealer, it is part of the specified finish, not an afterthought.
Underneath all of it sits cleaning, degreasing, and surface preparation, and this is where finishing quality is really won or lost. A coating is only as good as the surface it goes onto. Oils, soils, scale, and machining residue all have to come off first, through processes such as vapor degreasing and blasting, or the coating will not adhere or perform. If a finisher is casual about prep, be cautious, no matter what else they promise.
04.
Specifications, Testing, and Why Documentation Is the Point
Aerospace finishing is not about making a part look right. It is about proving the part meets its requirement, in writing, every time.
Your drawing will usually point to a specification, and those specs are written in a shorthand of MIL, AMS, and ASTM numbers. You do not need to memorize them. You need to understand that each one defines the process, the acceptance criteria, and often the testing. A good finisher reads those callouts the way you read a print, and can tell you when one is outdated or unclear.
Depending on the finish and the drawing, testing may include:
Salt-Spray Testing
Salt-spray testing, commonly to ASTM B117, to confirm corrosion resistance by exposing parts to a controlled salt fog for a set duration.
Coating-Weight Verification
Coating-weight verification, to confirm a phosphate or similar coating falls within the specified range.
Water-Break Testing
Water-break testing, a simple but telling check that a surface is clean enough to finish. Water sheets evenly on a clean surface and beads on a contaminated one.
Hydrogen-Embrittlement Considerations
Hydrogen-embrittlement considerations, which matter for high-strength steel. Acidic cleaning and certain processes can drive hydrogen into the steel, and in high-strength parts that can lead to delayed cracking. Where the drawing requires it, that means an embrittlement-relief bake after processing, and in some cases testing to ASTM F519. This does not apply to every part. It applies to specific materials and strength levels, and knowing which is part of the job.
Just as important as the tests are the records. Certificates of conformance, lot traceability, and documented process controls are what let you prove, months or years later, that a given lot was finished correctly. In aerospace, if it is not documented, it did not happen. This is also where Nadcap and ISO systems earn their keep, because they force the repeatability and record-keeping that make those certs trustworthy. It is the discipline JH Metal Finishing has built its reputation on across decades of specification-driven work.
One caution. Do not assume every job needs every test. Part of choosing well is understanding what your particular drawing and application actually require, so you are not paying for testing the part does not need, or worse, skipping testing it does.
05.
The Practical Purchasing Questions (FAQ)
Once you know what the part needs, the day-to-day questions get concrete.
What should I send to get an accurate quote?
Send the drawing, the material, the finish specification and any revision level, the quantity, and the end use if you can share it. The more the finisher knows, the tighter and more accurate the quote, and the fewer surprises later.
Does the drawing already dictate the finish?
Often, yes. If the print calls out a spec, that is the finish, and your job is to confirm the supplier can meet it and document it. When the callout is vague, outdated, or references a superseded spec, that is a conversation to have up front rather than a guess to make in production. A finisher who has seen the older specs come and go can usually tell you what the current equivalent is.
How do tolerances affect the choice?
Heavily. This is why black oxide, with its minimal buildup, gets specified on tight-tolerance parts where a thicker coating would push a dimension out of spec. Tell your finisher where the critical dimensions are.
Does the finisher need a particular accreditation for this part?
Come back to the first section. If Nadcap is flowed down, confirm the scope covers your process. If it is not required, you have more flexibility, but you still want documented process control.
How much should turnaround and location factor in?
They matter, but in their place. Technical capability, specification compliance, and reliability come first, always. Once a supplier clears that bar, proximity becomes a real advantage. A nearby finisher shortens the loop between machining, finishing, inspection, and assembly, reduces freight and handling risk on precision parts, and makes it far easier to walk a problem part across town instead of shipping it across the country. For a Connecticut shop moving parts through several operations, that is not a small thing.
06.
Aerospace Metal Finishing FAQs
What is the most important certification for aerospace metal finishing?
Before you evaluate price, turnaround, or anything else, confirm that the finisher holds Nadcap accreditation for the processes your part requires.
Why does Nadcap matter for aerospace finishing?
Nadcap is often a market-access requirement, not just a quality badge. If your program flows down Nadcap requirements and your finisher does not hold accreditation for that process, the part cannot ship, no matter how good it looks.
What metal finishing processes are common for aerospace parts?
Passivation, black oxide, manganese phosphate, zinc phosphate, and dry film lubricants each solve different engineering problems. The correct answer depends on the application, not habit.
How do I know if a finish will affect part tolerances?
Some coatings add measurable thickness while others do not. Tight-tolerance aerospace parts often require finishes selected specifically to preserve dimensional accuracy.
Why choose a Connecticut metal finishing supplier?
Shorter transportation distances can reduce freight costs, speed turnaround, simplify communication, and lower the chances of damaging precision components during shipping.
What should I send to get an accurate finishing quote?
Send the drawing, the material, the finish specification and any revision level, the quantity, and the end use if you can share it. The more the finisher knows, the tighter and more accurate the quote, and the fewer surprises later.
07.
Where This Leaves You
Choosing a finishing partner comes down to a short, honest checklist. Confirm Nadcap accreditation for the process you need. Make sure the shop reads and meets your specifications and can document that it did. Match the finish to the part rather than to a habit. And once all of that checks out, let proximity and responsiveness break the tie.
The truth about aerospace finishing is that it gets specific fast. Once the material, drawing, specification, tolerances, quantity, and end use are on the table, the right answer usually becomes clear, but it is rarely one-size-fits-all. That is where experience earns its place, and where a shop that has been doing this in Connecticut since 1953 can save you time and second-guessing.
So if you are staring at a drawing and not sure what it really requires, or you want to compare a couple of finishing options, or you just want a nearby finisher you can call when a question comes up, talk to JH Metal Finishing. Send the print, describe the part, and we will help you figure out what it actually needs.
Connecticut Aerospace Metal Finishing
Send Us Your Drawing
Need help confirming a finish, specification, testing requirement, or Nadcap-controlled process? Send the print and let JH Metal Finishing review the requirements with you.