Key takeaways
- A single powder coat at 100–120 µm covers C3 duty on pretreated steel; C4 at 160–240 µm is a primer and topcoat system, usually wet.
- Powder holds 40–60% of face film on a sharp edge and wet paint 60–75%, so a C4 part with unradiused edges needs the wet system or an R1.0 radius.
- A powder cure at 180–200 °C for 10–15 minutes rules out bonding, rubber inserts and thin heat-treated parts; wet paint at 60–80 °C does not.
- Colour change costs 20–40 minutes in a powder booth against 5–10 minutes with wet guns, so batches under about 50 m² usually favour wet paint.
A genset canopy bound for a coastal site and a rack panel bound for a server room can carry the same drawing note — "powder coat, RAL 7035" — and only one of them will still look right after five years. The difference is not the colour. It is the film build the environment demands, the edge coverage the geometry allows, and the cure temperature the assembly can survive.
Powder versus wet paint is settled by four numbers before it is settled by preference: the corrosivity category from ISO 12944-2, the total dry film thickness that category needs, the cure temperature the part can take, and the batch size over which a colour change stops mattering. Get those four right and the finish choice follows. Get them wrong and the part is coated twice.
This guide uses a 2 mm CRCA canopy side panel in C3 and C4 duty as the worked case, with a 190 °C powder cure and a 70 °C wet bake as the two routes.
Which finish meets the duty?
A single powder coat at 100–120 µm on conversion-coated steel covers C3, the urban and industrial inland category. C2, indoor and sheltered, is satisfied at 60–80 µm. C4, which covers coastal sites and de-icing salt, needs 160–240 µm total and is normally a primer plus topcoat, which is wet work or a two-coat powder system. C5 goes further again and is rarely a single-coat answer in any process.
That is the decision rule in one paragraph: single colour, C3, part fits the oven, no heat-sensitive elements — powder. Multi-coat build, C4 or worse, metallic or special effect, part too large for the oven, or a finish that must be touched up in the field — wet.
The reason is film build per pass. Powder lays down 60–100 µm in one application and one cure. A wet primer at 40–60 µm plus a topcoat at 40–60 µm gets to 80–120 µm in two passes, and a C4 system at 160–200 µm needs either a high-build primer or three coats. Each process can reach any build with enough passes; the question is how many passes the part can economically take.
Powder versus wet paint: the comparison table
| Parameter | Powder | Wet paint | Ganesh limit or note |
|---|---|---|---|
| Film per pass | 60–100 µm | 30–60 µm | C4 needs 160–240 µm total, so wet is usually two or three passes |
| Edge coverage, sharp 90° | 40–60% of face | 60–75% of face | R1.0 radius lifts powder to about 95% |
| Cure schedule | 180–200 °C, 10–15 min | 60–80 °C bake, or air dry | Powder heat-soaks the whole part |
| Colour change | 20–40 min including reclaim | 5–10 min gun flush | Batches under about 50 m² favour wet |
| Transfer efficiency | 60–70%, up to 95% with reclaim | 65–75%, overspray lost | Reclaim is why powder suits long runs |
| Orange peel at 60° gloss | Visible at 60–100 µm | Low at 35–50 µm | Cosmetic panels often specify wet |
| Metallic and effect colours | Limited | Full range | Bonded metallic powders exist, colour range is narrower |
| Repair after installation | Not invisible | Matched touch-up | Site damage favours wet |
| Heat-sensitive assemblies | Not possible | Possible | Adhesive, rubber, seals and thin spring parts |
| Maximum part envelope | 18 ft × 14 ft booth | Booth and oven dependent | Longer parts need a wet route |
| DFT measurement | ISO 2808 magnetic | ISO 2808 magnetic or eddy current | Nine points per part, recorded |
| Salt spray target | 480–720 h NSS typical for C3 | 720–1,000 h NSS for C4 systems | ISO 9227 neutral salt spray, creep measured from the edge |
| VOC and solvent handling | No solvent in the film | Solvent or waterborne | Waterborne reduces solvent, needs controlled drying |
| Recoat over existing film | Powder over cured powder, abrade first | Wet over most cured systems | Unknown prior paint needs a test patch |
The Ganesh column reflects what the plant can actually run. The powder booth is 18 ft × 14 ft, so a canopy longer than that envelope has to be wet painted or made in sections. Seven-tank pretreatment, a wet booth, a curing oven and DFT and cross-hatch checks sit on the same line, which is why the route can change without changing supplier.
Why edge coverage splits the two processes
Powder is a dry, charged particle that lands, melts and flows. Surface tension pulls the molten film away from a sharp arris, and the electrostatic field is weakest at a 90° corner because there is almost no grounded area facing the gun. The result is 40–60% of face film at a sharp edge.
Wet paint arrives as a liquid with much lower surface tension and wets the edge before it dries. It still pulls back, but less: 60–75% of face film at the same corner. That difference is what makes a C4 system achievable on a laser-cut edge without a radius, and it is why a powder specification at C4 often fails at the edge while the face measures perfectly.
The film numbers matter because corrosion starts at the edge. At 80 µm face powder, a sharp edge carries 32–48 µm — below the 60 µm that a C3 system assumes at the steel. Radius the edge to R0.5 and powder retention rises to 80–95%; radius to R1.0–2.0 and it is effectively full thickness. The radius is a laser-path change and costs almost nothing. The extra film to compensate costs powder on every part and cracks on assembly.
The 0.8 mm problem
Thin sheet changes the calculation in a different way. A 100 µm powder film on 0.8 mm CRCA is stiff relative to the panel, and the film cracks when the part is flexed during installation or when a fastener is torqued. Wet paint at 60–80 µm total flexes with the sheet. Where the part is a cover rather than a structural panel, that flexibility is worth more than the powder durability.
Film build and bend radius
Coating after forming is normal, but a part that is bent after coating puts the film in tension on the outside of the bend. Powder at 80 µm cracks at a bend radius below about 2t on 1.5 mm sheet; wet at 50 µm survives about 1t. If the process requires post-coating forming, either keep the bend radius generous or specify a flexible wet system.
What the cure temperature rules out
The powder cure is the hidden constraint in assembly design. At 190 °C for 12 minutes, the whole part reaches oven temperature, not just the surface. Anything in the assembly that cannot take that soak has to be added after coating:
- structural adhesive joints, which soften at 40–60 °C glass transition for a toughened acrylic;
- rubber gaskets, cable glands and sealing strips;
- plastic inserts, threaded bushes and cable ties;
- pre-tinned or soldered joints, which reflow;
- thin springs and clips that lose temper above 150 °C;
- zinc-plated fasteners, where the zinc can diffuse into the steel at 190 °C and the sacrificial protection changes.
Wet paint at 70 °C bake removes that constraint. It is the reason a fully assembled genset canopy with wiring, foam and a bonded liner is painted wet, and the reason a bare steel canopy frame is powder coated before assembly.
The counter-argument is durability. A wet system relies on the primer for adhesion and the topcoat for UV and colour, and each coat is a chance for a defect. A single powder coat has one interface and one cure, and on C3 duty it is the more repeatable process.
Pretreatment is common to both
Neither finish works without conversion coating. The seven-tank line runs alkaline clean, rinse, activation, zinc phosphate or zirconium conversion, rinse, demineralised rinse and dry-off. Coating weight is checked gravimetrically to ISO 3892 on a coupon, and the dry-off oven has to remove water without over-drying the conversion layer.
A powder part and a wet part leave the same pretreatment line. That is why changing finish mid-project is a coating change, not a change of supplier — provided the drawing's DFT and test requirements are updated with it.
Where powder wins
Long runs in one colour. Reclaim recovers 30–35% of overspray, so consumption settles at 130–160 g/m² at 70 µm. A wet line at 60% transfer efficiency sends the rest to the filter.
Single-coat C3 duty at 100–120 µm. One application, one cure, one interface. The finish is harder, more abrasion resistant and less sensitive to operator technique than a two-coat wet system.
Parts with recessed geometry. Powder's electrostatic wrap reaches into box sections better than a spray gun, though neither coats a 10 mm wide, 20 mm deep channel at full film.
Low VOC requirement. Powder contains no solvent. Where the customer's environmental permit or a green building specification limits VOC, powder is the simpler route.
Where wet paint wins
C4 and C5 systems. A 200 µm build in a primer and topcoat is standard wet work. Two-coat powder can reach it, but the second coat needs its own cure and the intercoat adhesion has to be proved.
Colour range and gloss control. Deep colours, metallics, pearlescents, matt at 5 units and gloss at 90 units are all wet-paint territory. Powder gloss is limited by the resin and the flow.
Large parts. The 18 ft × 14 ft powder booth sets a hard envelope. A 7 m canopy panel cannot go through it, and a wet booth with a track can handle it in sections or as a whole.
Field repair. A scratched powder part cannot be made invisible. A wet part can be touched up with a matched kit, which matters on installed equipment.
Assemblies with heat-sensitive content. Wiring, foam, gaskets, adhesives and plastics all survive a 70 °C bake and none survive 190 °C.
What Ganesh actually runs
Coating here starts at the seven-tank pretreatment line, which serves both finishes. Powder goes through the 18 ft × 14 ft electrostatic booth and the curing oven; wet paint has its own booth and bake. Parts that need an anchor profile before either route go through the 16 ft × 16 ft shot blast chamber, and every batch is checked with a DFT gauge and a cross-hatch adhesion test.
The paint shop capability page sets out the line, and seven-tank pretreatment explains the stages that both finishes depend on. We do not hot-dip galvanise in-house; where the specification calls for a duplex system over galvanising, the galvaniser sits in the chain and the powder or wet coat has to be compatible with a zinc surface.
What to put on the drawing
- Corrosivity category from ISO 12944-2, stated as C2, C3, C4 or C5, not implied by the word "outdoor".
- Total dry film thickness with a minimum and maximum, and the measurement standard (ISO 2808).
- Primer and topcoat requirement, if the system is multi-coat, including the colour of each.
- Colour standard, RAL or a physical master, with the gloss level at 60°.
- Edge condition, radius and burr limit, since both processes lose film there.
- Salt spray hours and acceptance, measured to ISO 9227 with creep from the edge, not from a face scribe.
- Cure temperature limit, if the assembly carries heat-sensitive parts, so the coater selects the process rather than discovering the problem at the oven.
- Touch-up requirement, if the part will be installed on site and scratched.
For the failure side of this decision, why powder coating fails on outdoor enclosures covers the mechanisms, and powder coating laser-cut edges goes deeper on edge preparation. The surface treatment hub indexes the rest. If the finish route is still open, send the duty and the drawing to request a quote and both routes can be priced against the same part.
When neither finish is right
Hot-dip galvanised and unpainted. Where the part is a structural frame in a wet environment and appearance does not matter, galvanising to ISO 1461 gives a zinc coating of 45–85 µm depending on thickness and is cheaper than paint plus maintenance. Powder over galvanising needs a proper pretreatment or the zinc surface will outgas and blister.
Zinc-rich primer only. For a part that will be overcoated on site, a zinc-rich primer at 60–80 µm is a holding coat, not a finish.
E-coat. Where the acceptance criterion is edge and cavity coverage — a box section, a closed frame, a part with an internal weld — electrophoretic coating deposits 20–25 µm uniformly on every wetted surface. Neither powder nor spray wet paint reaches inside a closed box, and no gun setting changes that.
No coating at all. Stainless and aluminium parts in indoor duty often need nothing beyond a brushed or anodised finish. Adding powder to a 304 cover in a server room adds cost and a film that can chip, with no corrosion benefit.
Frequently asked questions
Is powder coating better than wet paint for outdoor sheet metal?
For a single colour in C3 duty, yes: 100–120 µm of polyester powder on a conversion-coated surface is a durable finish, and it avoids the solvent in a wet system. For C4 coastal or C5 industrial duty the specification calls for a primer and topcoat at 160–240 µm, and a wet system reaches that build in fewer passes.
Can I powder coat a part that already has a bonded insert?
No. The 180–200 °C cure for 10–15 minutes softens most structural adhesives and destroys rubber and plastic inserts. Coat the steel parts first, then bond or assemble. If the insert is unavoidable, use a wet system that cures at 60–80 °C.
How thick is powder coating on an edge?
A sharp 90° edge holds 40–60% of the face film, so an 80 µm face leaves 32–48 µm at the arris. Wet paint holds 60–75%, and a R1.0 radius lifts powder retention to about 95%. Radius the edge before adding film.
Can powder coating be touched up on site?
Not to an invisible repair. Powder is cured as a continuous film, and a field repair is a wet touch-up pen or aerosol that will differ in gloss and colour. Where the part will be scratched in installation, wet paint with a matched touch-up kit is the practical choice.
Why does powder look more textured than wet paint?
Powder is applied at 60–100 µm and flows for 10–15 minutes; the film is thick enough that the surface does not level completely, so orange peel is visible at 60° gloss. Wet paint at 35–50 µm levels flatter. A thin powder coat at 60 µm is smoother but covers edges and profile less.
Does colour change cost more with powder?
Yes, in small batches. Cleaning the booth, the reclaim system and the gun path takes 20–40 minutes between colours, and reclaim material from the previous colour is lost. Wet guns flush in 5–10 minutes, which is why a 20-part order in three colours is usually painted wet.
Sources and further reading
- ISO 12944-2:2017 — Corrosion protection of steel structures: classification of atmospheric corrosivity from C1 to CX · reviewed 3 Nov 2026
- ISO 12944-5:2019 — Protective paint systems: primer and topcoat build-ups by corrosivity category and durability · reviewed 3 Nov 2026
- ISO 2808:2019 — Paints and varnishes: determination of film thickness, the magnetic and eddy-current methods used for DFT checks · reviewed 3 Nov 2026
Send the duty, not just the colour
Give us the service environment, the colour standard and any salt spray target. We will confirm whether powder or a wet system meets it and quote the route that does.
Request a coating quote