Powder coating is the cheapest way to give a metal part a durable, uniform finish, and the fastest way to lose a batch if the specification says only “black”. The coating is applied dry and cured into a film, so its appearance and performance depend on the chemistry, the film thickness, the texture and the pretreatment, all of which are decided long before the parts reach the line. This guide covers what a powder coating specification should contain, how chemistries and colour systems differ, what drives cost, and how to write an acceptance criterion for a finish that will be judged by eye.
What should a powder coat specification include?
Chemistry, colour reference, texture, thickness and masking.
A workable specification names the powder family, a colour reference, the sheen or texture, the film thickness range, and which features stay bare.
Each item answers a question the finisher would otherwise have to guess. The powder family determines durability and the environment the finish suits. The colour reference, whether an RAL number, a physical sample or a Pantone match, removes the ambiguity that descriptions create. Texture and sheen set the appearance, and they are produced by the powder rather than by the application. The thickness range affects both appearance and the dimensional consequences. And the masking list determines which features are protected from a coating that will otherwise bridge threads and close clearances.
Two further items are worth adding on any part that will be inspected on arrival. The pretreatment the substrate requires, particularly on aluminium, and the acceptance criteria, which should name how colour and appearance are judged. A physical reference part, produced and approved once, is the most reliable of those criteria, for the same reason it works in anodizing.
Writing the specification this way also makes quotes comparable. Two powder coating quotes that assume different thicknesses, different pretreatment or different masking are not the same product, and the difference is usually invisible until the parts arrive. The manufacturing quality practices that support a repeatable finish are described by NIST MEP.
Which powder chemistry fits which environment?
Choose by exposure, not by colour.
Epoxy powders bond well but chalk outdoors, polyester and hybrid powders are the general-purpose choices for exterior parts, and specific chemistries address heat or chemical exposure.
The chemistry determines how the film behaves over time. Epoxy-based powders produce a tough, chemically resistant film with excellent adhesion, and they are a common choice for indoor parts and equipment. Their weakness is ultraviolet exposure: an epoxy film chalks and fades in sunlight, which is why they are not specified for outdoor products.
Polyester and hybrid chemistries are the general-purpose answer for parts that will be seen outdoors. They hold colour and gloss far better under exposure, and they cover the majority of architectural, furniture and equipment applications. Where a part will be exposed to high temperature, a silicone-based or specialist powder is used, and where chemical resistance is the governing requirement the discussion returns to epoxy or a specialty formulation.
The practical implication is that a colour match across two different chemistries is not achievable, because the same dye or pigment behaves differently in each film. Where a program includes indoor and outdoor parts in the same colour, the two should be specified in the same chemistry even if that costs slightly more on one of them. The test methods that describe coating performance are published by ASTM committee B08 on metallic and inorganic coatings.
How do colour and texture systems work?
Colour standards set the shade; the powder sets the finish.
RAL and similar colour systems give a reference for the shade, while texture and gloss come from the specific powder, so both have to be named for the result to repeat.
Colour references are the easy part. A RAL number is a widely recognised way to name a shade, and most powder manufacturers publish their nearest equivalent. Pantone matching is also common where a product colour has to match packaging or another component. Both are approximations in a dry film, which is why a physical sample or a sprayed test panel is the practical way to approve a colour before a batch runs.
Texture is the part that surprises buyers. Powders are available in smooth, satin, matte and structured finishes, and the same colour in two textures reads completely differently. A structured powder, which produces a fine wrinkle pattern, hides surface imperfections and is often used on equipment and enclosures for that reason. A smooth high-gloss powder shows every variation in the substrate, which makes it unforgiving on welded or formed surfaces.
The two decisions interact with the part’s geometry. Where a surface has welds, forming marks or a coarse finish, a smooth gloss film will reproduce them faithfully, whereas a textured film will disguise them. Choosing the texture with the substrate in mind is cheaper than improving the substrate to suit the finish.
How do thickness, coverage and edges behave?
Thickness drives durability and dimensional change together.
A thicker film resists corrosion and abrasion better, closes the clearance on threads and bores further, and costs more powder and time, so the thickness range has to be specified rather than assumed.
Film thickness is measured in thousandths of an inch or in micrometres, and the range that suits a part depends on the environment and on the geometry. A film at the lower end of a commercial range may cover adequately but offer less corrosion protection on edges; a thicker film protects better but is more prone to edge build-up and to closing clearances on features that must still function.
Edges behave differently from flat surfaces because electrostatic attraction pulls powder toward them, which builds a thicker film along a sharp edge and can cause a visible ridge or, in extreme cases, a crack when the film cures. Where a part has many sharp edges, a radius or a deburring step reduces the effect and produces a more uniform appearance.
Coverage in recesses and shadowed areas is the other consideration. Powder is applied by spray and attracted electrostatically, so an area that is difficult to reach may receive less material. Where a part has deep pockets or re-entrant geometry, that should be raised at quoting stage, because the alternative is a film that looks finished on the outside and is thin or missing inside.
| Item | What it controls | What happens if omitted |
|---|---|---|
| Powder chemistry | UV stability, chemical resistance, heat tolerance | A finish that suits the wrong environment |
| Colour reference | Shade target | An approximation that is argued about later |
| Texture and gloss | Appearance and how much the substrate shows | A finish that reads differently than expected |
| Film thickness | Durability and dimensional effect | Clearances that close more than planned |
| Pretreatment | Adhesion and corrosion performance | Premature adhesion failure |
| Masking list | Which features stay bare | Coated threads and blocked holes |
How does pretreatment affect the result?
It determines whether the film stays on.
Cleaning and conversion coating prepare the metal so the powder bonds, and a shortcut at this stage shows up months later as peeling, blistering or corrosion under the film.
Pretreatment is the least visible and most important part of powder coating. The substrate has to be free of oil, scale, oxides and handling contamination before the powder is applied, which typically means a cleaning stage followed by a conversion coating appropriate to the metal. Steel, aluminium and galvanised substrates each need different chemistry, and the conversion layer is what allows the film to bond and what limits corrosion that starts at a scratch.
The consequences of inadequate pretreatment are delayed and expensive. A film applied over contamination may look perfect on delivery and lift at a scratch after a few months outdoors, and the failure is usually discovered in the field rather than in inspection. That is why the pretreatment method belongs in the specification for any part that will see weather.
Substrate condition before pretreatment matters too. Weld scale, rust and forming lubricants all need to be removed, and where a part has been stored for a period, the cleaning stage has to account for the surface it has become. Surface preparation practice is described in ASTM D3359, which covers the tape test used to assess adhesion, and the coating framework is published by ASTM committee B08.
What drives the cost per part and per batch?
Labour, batch size and colour.
Powder coating prices are dominated by handling: parts are hung, masked and inspected individually, so a batch with many small parts or a masking requirement costs more per unit.
Racking and hanging set the batch size. Parts have to be hung so that powder reaches the surfaces that need coating and so that the film is not disturbed before curing. That limits how densely a batch can be loaded, which in turn determines how much labour each part carries.
Masking is the second cost. Each protected feature has to be plugged, capped or taped before coating and cleared afterwards, so a part with several masked threads costs more to finish than the same part with none. Where a design allows a thread to be protected by a plug that stays in place through curing, the process is simpler; where a close-tolerance bore has to be kept bare, the work is more delicate.
Colour and chemistry come third. Standard colours in a common chemistry process in batches with other work; special colours and small quantities may require a dedicated run with a minimum charge. Thickness requirements matter as well, since a specified range at the top end consumes more powder and more time and is more likely to require a second pass.
The practical consequence is that powder coating rewards a part designed for the process. Radii instead of sharp edges, a masking list limited to the features that genuinely need protection, and a colour chosen from a standard range all reduce cost without touching the product’s appearance. Requests submitted through the quote flow receive a review so those items are settled before the parts are run, and any coating line waste is handled under the framework published by the US Environmental Protection Agency.
What should be checked, and what can be touched up?
Thickness, adhesion and appearance against a reference.
Inspection covers film thickness on the part, adhesion where it matters and appearance against an approved sample, and the touch-up policy should be agreed before delivery.
Thickness is measured with a gauge on the finished part and compared with the specified range. Adhesion is assessed with a tape test or a similar method, usually on a sample panel rather than on the delivered parts where the test is destructive. Appearance is judged against the approved reference under a defined light, which is the only way to make a colour decision repeatable.
Touch-up is a practical question that deserves an answer before a batch ships, because small defects are common in any coating process. A touched-up area is repaired paint rather than cured powder, so it behaves differently and looks different under close inspection. Where a part is cosmetic, the acceptable level of touch-up should be defined; where a part is functional, a repair is often acceptable without further discussion.
6CProto provides quality inspection reports on request and assigns a dedicated project manager to each order, so the acceptance criteria for a coated batch can be agreed with the finishing specification. Process waste from the coating line is handled under the framework published by the US Environmental Protection Agency, and the quality practices that support finishing qualification are described by NIST MEP.
Specifying a finish that repeats
Powder coating is a process with a small number of variables, and each one can be named in a specification: chemistry, colour reference, texture, thickness, pretreatment and masking. Where all six are stated, the finish repeats across batches; where any is left to interpretation, the batch is approved or rejected on the finisher’s judgement rather than the buyer’s requirement.
Two habits make the difference in practice. Approve a sprayed sample panel in the production chemistry before the batch runs, since colour and texture are judged by comparison. And keep the masking list short and deliberate, because every protected feature is hand work and every unnecessary one is money spent on a problem the part did not have. The alternatives to powder coating, and how they compare for the same part, are covered in the 6CProto article on choosing between powder coating, anodizing and plating.
FAQ
How much should powder coating cost?
Price is driven by handling rather than by powder. Parts are hung individually, masked by hand where features need protection, and inspected before packing, so the cost per part falls as batch size rises and as the masking list shortens. Colour choice matters too: a standard shade processes with other work, while a special colour may require a dedicated run. A quote that states thickness, pretreatment and masking is the one worth comparing.
What are the downsides of powder coating?
Three are worth planning for. The film adds thickness, which closes clearances and bridges threads unless those features are masked. Sharp edges attract extra powder and can build a visible ridge or crack during curing. And the finish depends on pretreatment, so a shortcut there produces a film that looks correct and fails later. None of these is a reason to avoid the process, but each belongs in the specification.
Is powder coating cheaper than painting?
For metal parts produced in quantity, it usually is, because the coating is applied in one pass and cures quickly, and because there is no solvent to handle. The comparison depends on the part: a small batch in a single colour is straightforward to powder coat, while a multi-colour product may need masking between colours and can become labour-intensive. For a single part in a non-standard colour, liquid paint can be more practical.
How long does a powder-coated finish last?
Durability depends on the chemistry, the pretreatment and the exposure. A polyester or hybrid powder over a properly prepared substrate holds its colour and gloss well outdoors for years, while an epoxy powder will chalk under the same exposure because it is not UV stable. In a sheltered or indoor environment, most chemistries last far longer. Specifying the chemistry against the environment the part will see is the main factor under the buyer’s control. The test methods referenced in this article are published by ASTM committee D20.
If metal parts need a coating that survives the environment they will see, send the model with the colour reference, the texture and the features that must stay bare. 6CProto reviews the part together with the finishing route and returns a DFM report with the quote, so chemistry, thickness and masking are agreed before the batch runs. Upload the file at the 6CProto quote page or send it to projects@6cproto.com.