Guides / Choosing a Supplier

How to Choose a Metal Coating Supplier: What to Check Before You Send Parts

September 23, 2026

Choosing a metal coating supplier is mostly about fit, not reputation. The right shop already runs your process, holds your spec, has a tank or line big enough for your part and can hand back the paperwork your customer expects. Get those four right and price and lead time usually fall into place. Get one wrong and you find out when the first lot comes back peeling, oversize or without a cert.

This guide walks through what to check, in the order that saves the most time.

Start with the process, not the shop

Before you look for anyone, pin down what the finish must do. Corrosion protection on a steel bracket, wear resistance on an aluminum bore, electrical contact on a busbar and a cosmetic black on a handle are four different jobs, and they lead to four different kinds of shop.

Write down three things:

• The base material and its condition. “4140, heat treated to 42 HRC” is far more useful than “steel”.
• The job the finish has to do, in plain words.
• Any spec your customer or your own drawing already names.

If you are not sure which finish family fits, the finish comparison guide and the self-check on the home page give you a starting point. Shops will refine it, but they need to know what problem they are solving.

Check that the shop runs your spec, not just the process

Plenty of shops “do anodizing”. Fewer run MIL-PRF-8625 Type III with the process control and test coupons that spec expects. The same goes for zinc-nickel to ASTM B841, electroless nickel to AMS 2404 or passivation to ASTM A967.

Ask each shop directly:

• Which specs do you run every week, not just occasionally?
• Can you show a recent certificate of conformance for this spec with the customer name removed?
• For controlled specs, do you hold the approval your customer needs, such as a customer source approval or Nadcap chemical processing accreditation?

A shop that runs your spec weekly has its tank chemistry, thickness checks and records already set up for it. A shop that runs it twice a year is learning on your parts.

Match the part to the tank, barrel or line

Size limits decide more finishing quotes than anything else. Every shop has a largest tank, a barrel capacity or a line envelope, and a part that does not fit is either refused or split across awkward runs.

Give the shop the overall size and weight of your largest part and ask how it will be held:

Handling	Suits	Watch for
Barrel	Small parts in bulk: screws, pins, small stampings	Contact marks, flat parts sticking together
Rack	Larger, heavier or cosmetic parts	A small mark where the part hangs
Conveyor line	Powder coat and e-coat	Needs a hang hole or hook point

If your part is cosmetic, ask where the rack or hook contact will be and mark an acceptable spot on the drawing. That one note prevents most cosmetic rejects.

Ask about pretreatment and what happens before the tank

Most coating failures start before the coating. Oil, rust, scale, heat treat oxide or old paint left on the part leads to poor adhesion, blisters or bare spots.

Ask the shop:

• How do you clean and activate this base material?
• Do you handle scale from heat treatment, or should it be blasted first?
• For stainless or aluminum, what activation step do you use before plating?

Also tell them what is on the part when it arrives. Cutting oil from machining, rust preventive from storage and weld spatter all change the cleaning route and the price.

Hardened steel needs a bake plan

If any part is high strength steel, typically above about 39 HRC, electroplating and acid cleaning can put hydrogen into the metal. Without a relief bake soon after plating, the part can crack under load later.

Put the requirement on the drawing, either by naming ASTM B850 or your own bake requirement, and ask the shop:

• How soon after plating does the bake start?
• What time and temperature do you use for this hardness?
• Is the bake recorded on the certificate?

If a shop treats this question as unusual, it is not the right shop for hardened parts. For hardened fasteners, ask about zinc flake or mechanical zinc, which avoid electrolytic hydrogen entirely.

Confirm masking in writing

Threads, precision bores, bearing seats and grounding points often have to stay bare or take a thinner coat. Masking is hand work, so shops price it separately and sometimes quote without it if the drawing is unclear.

Circle every masked area on the drawing and write what is allowed on it: bare, thin film only, or finish acceptable. Then ask the shop to confirm masking in the quote, line by line. It is the single most common reason two quotes for the same part differ by a factor of two.

Agree on inspection and paperwork before the first lot

Decide up front what proves the lot is good. Common items:

• Thickness readings by magnetic induction, eddy current or XRF, with the number of parts and points measured.
• Adhesion checks, such as a bend or tape test, when the spec calls for them.
• Salt spray per ASTM B117 on test panels or parts, if your customer requires it.
• Bake records for hardened steel.
• A certificate of conformance naming spec, type and class.

Paperwork you did not ask for is paperwork you will not get. Paperwork you ask for after the parts ship is paperwork the shop may not be able to recreate.

Look at capacity and lead time honestly

Finishing lead times are usually short once parts arrive, often days rather than weeks. The delays come from queues on a busy line, color changes on a powder line, masking backlogs and waiting for a bake oven.

Ask what the normal turnaround is for your lot size and what happens when volume doubles. If you have a repeating order, ask whether the shop can hold a regular slot for you.

Plan the logistics

Finishing sits between your machinist or stamper and your assembly line, so transport and packaging matter:

• Who ships parts in and out, and in what packaging?
• Will cosmetic parts be packed individually after coating?
• If parts come straight from your machinist, can the shop inspect incoming parts and report damage before coating?

Rough handling between steps causes as many rejects as the coating itself.

Questions to send every shop

Copy this list into your RFQ email or attach it to the drawing:

1. Which of these specs do you run weekly?
2. What is your largest tank, barrel load or line envelope?
3. How will this part be held, and where will the contact mark be?
4. What cleaning and activation do you use for this base material?
5. Can you bake hardened steel, and is the bake on the cert?
6. What will you measure on each lot, and what do you send back?
7. What is your normal turnaround for this lot size?

Shops that answer all seven clearly are worth a trial lot. Shops that skip the hard ones usually tell you something too.

How we help

We are a matching service, not a plating shop. You send one RFQ with the drawing, base material, finish and spec, and we pass it to up to three shops that run that process, hold the spec and have a line that fits the part. They quote you directly. If you want to write the RFQ so shops can price it first time, the coating RFQ checklist covers every field, or you can go straight to the RFQ form.

See also how to check a finishing shop’s approvals.

Frequently asked questions

Keep reading

• Questions to Ask a Plating or Coating Shop Before You Send Parts
• RoHS and REACH Compliance for Plated and Coated Parts: What to Ask For and What the Tests Show