Powder Coating Film Thickness: Ask for 80-100 Microns
by Argent Gates
Aluminium does not rust. So why coat it?
Aluminium grows an oxide film the moment it meets air, and that film is self-limiting: it seals the surface and stops itself growing. That is standard materials science, not a sales claim. It is why bare aluminium does not flake apart the way bare steel does. Our stated figure for welded 6063-T5 structures is 30 years without corrosion or rust-through (that is the expected life of the alloy and the weld, not a record we have watched run out — the works dates to 2005).
The coating is not there to save the metal. It is there for colour, gloss and UV stability, and to hold off cosmetic corrosion in coastal or de-iced-road air. Our stated figure for the coating is 10-15 years without fading. Put the two numbers side by side and the point is obvious: the finish has a shorter service life than the frame it sits on.
So film thickness is not a detail. It is the variable that decides how long the gate keeps looking like the gate you bought.
What happens on the line: pretreatment, spray, cure
Pretreatment comes first: degrease, rinse, etch, conversion coating, rinse, dry. This stage decides adhesion. Most coating failures on aluminium start here — a contaminated surface, exhausted chemistry, dirty rinse water — and no powder, at any price, rescues a bad pretreatment.
Then electrostatic spray. Powder is charged and thrown at an earthed part; it clings by charge, not by wetting. Flat faces coat easily. Recesses do not. The charge field struggles to reach inside corners, tight picket gaps and the backs of scrollwork. That is the Faraday cage effect, and those are the spots where a decorative gate runs thin.
Then cure. The part goes through an oven, the powder melts, flows and cross-links into a continuous film. Two things follow from that: the film only exists after the oven, and powder cannot be re-cured outside one.
Where we sit in this: six steps are in-house — cutting, welding, grinding, assembly, pretreatment, spraying — plus hand gilding. The powder is bought in: TIGER Coatings, Austria, super-durable exterior grade, specified at 80-100 µm. The profiles are bought in too. We do not extrude. Primary 6063-T5 comes from Anhui Guangyin Aluminium, a state-owned brand, and Jiangxi Huike. Worth saying plainly, because a factory that owns pretreatment and spray owns the coating failure. When both are subcontracted, nobody does.
Too thin chalks, too thick chips
Below the band, UV works through the binder from the surface down and reaches the far side sooner. You get chalking — a dull powdery bloom — and colour drift. A thin film is also a thinner moisture barrier, so corrosion creeps out from cut edges and drilled holes under the coating.
Above the band, the film gets harder and more brittle. Impact resistance drops and edges and corners chip, which is exactly where a gate takes knocks. Very heavy films can also sag or orange-peel. 80-100 µm is a common target for super-durable exterior powder because it clears the UV problem without walking into the brittleness one.
One caution: the band is a range across the whole part, not a single reading. A leaf that averages inside the band can still carry an inside corner well below it, and that corner is where it will fail. Averages hide the failure point.
| Film condition | What goes wrong | Where it shows first |
|---|---|---|
| Below the target band | UV reaches the binder sooner; chalking and colour drift; weaker moisture barrier | Sun-facing flat panels; cut edges and drilled holes |
| 80-100 µm | Intended case: full UV barrier, film still flexible | - |
| Above the target band | Harder, more brittle film; impact resistance drops; possible sag | Edges, corners and latch areas, on impact |
How to verify film thickness on a delivered gate
Use the right gauge. Magnetic-induction gauges read nothing useful on aluminium, because aluminium is non-ferrous. You need an eddy-current gauge, or a dual Fe/NFe gauge switched to non-ferrous. They are inexpensive and a leaf takes a minute.
Measure where the film is likely to be thin, not where the gauge sits comfortably. Inside corners, gaps between pickets, the back face of the frame, behind cast ornament. Take several readings per leaf, then judge by the lowest one.
Adhesion is a separate question, and thickness does not answer it. The cross-hatch test does: cut a lattice through the coating down to bare metal, press adhesive tape over the cuts, pull it off, and see how much coating lifts with the tape. It is destructive, so run it on an offcut or a hidden face. It tests the pretreatment.
We can supply third-party test reports, and we should be exact about what they are: they are issued by Chinese domestic third-party institutions. If your specification calls for a European or Australian laboratory, that is a separate exercise. Plan for it rather than assuming our report substitutes.
Why "we use imported powder" proves nothing on its own
A brand names a supplier, not a product. Powder makers sell standard polyester for sheltered use and super-durable grades for exterior weathering under the same name on the box. A brand without a grade tells you nothing about UV performance.
Even the correct grade fails at the wrong thickness or over poor pretreatment. Powder is a small share of the finished cost, and the invisible steps are where cost quietly comes out. A brand name also cannot be checked in a photograph. A film thickness can be checked in a minute with a gauge.
Ask for three things in writing: the powder grade, the target dry film thickness range in microns, and the alloy and temper. Ours: super-durable exterior powder from TIGER Coatings, Austria; 80-100 µm; primary 6063-T5, with a 3.0 mm wall on gate frames. That 3.0 mm figure is the gate frame specifically — railing sections are a different specification, and any supplier quoting one wall thickness for a whole catalogue is rounding.
And the limit on our own number: we quote 10-15 years without fading for the coating, but our first export shipment was in 2020. That is a few years of overseas field history, not fifteen. The figure describes what a super-durable exterior system at this thickness is expected to deliver. It is not a fifteen-year field record we can show you, and neither can most of the factories quoting it.
| What a supplier says | What it actually proves | What to ask instead |
|---|---|---|
| "We use imported powder" | That a brand was named | Which grade - standard, or super-durable exterior? |
| "It is super-durable" | The resin class, at best | What is the target dry film thickness, in microns? |
| "10-15 years, no fading" | The expected performance of that coating class | Is that the class expectation, or measured field data? |
| "We have test reports" | That a test happened somewhere | Which lab, which country, which test, what date? |
What the coating cannot survive: shipping and site assembly
Everything you receive was coated before it left the factory, because powder cures in an oven. There is no field re-cure. Any repair on site is liquid paint, and it will not match the gloss or age at the same rate.
Design around that. Our largest single leaf is 7 × 4 m, which is an upper limit and rarely ordered. Anything past shipping dimensions is broken into sections at the factory, coated as sections, and joined on site. Site work should be bolted, never cut or welded: a cut exposes bare aluminium, and a weld burns the coating off both sides of the bead. Boxes ship with the full screw set, and local fitters handle it. Motors and locks are optional at extra cost.
If a section genuinely has to be trimmed on site, treat it as a compromise and budget for touch-up. Better to say that before the container ships than to discover it at the gate post.
Film thickness is the one claim in a coating specification you can put a gauge on. Ask for the number in writing, then measure it when the crate opens - and treat any supplier who will not give you a range as having answered the question.