Salt Spray, MEK Rubs, Gloss: Three Tests That Separate Good Coils From Bad

April 17, 2026 | Tech Notes

Your eyes cannot tell you how a coil will look in three years. People in the trade judge a coil by three test reports: salt spray, MEK rubs and gloss. None of them needs a lab degree to read. Here is what each number means.

Test 1 — Salt spray: how long before rust

Samples go into a chamber at 35 °C with a continuous 5% salt fog. The report says when white rust and red rust appear.

Construction grade: 200–300 hours is normal
Appliance grade: 500 hours and up — higher zinc holds longer
What to read: rust creep from cut edges and the scribe mark, not surface blisters

Buyer beware: the report must state the zinc coating. 500 hours on Z120 and 500 hours on Z60 are not the same claim. An hour count without a zinc weight is meaningless.

Test 2 — MEK rubs: is the paint baked through

MEK (methyl ethyl ketone) is an organic solvent. A cloth soaked in MEK wraps a 1 kg hammer and rubs the coating back and forth; you count the double rubs until the film wears through to metal.

50 rubs: under-cured — the film powders and flakes in stamping
100+ rubs: fully cured — T-bends and cupping tests hold
100 is the threshold for stamped appliance parts

This test is cheap and quick. A buyer can ask for it live: one sample, one bottle, ten minutes. It is more honest than any promise.

Flying shear on the cut-to-length line — Under-cured film fails right here — flaking starts at the cut edge

Test 3 — Gloss: will the batch look like one color

A 60° gloss meter reads the surface in GU (gloss units).

Appliance side panels typically run 20–60 GU, and variation on one assembly line should stay within ±5
Measure head and tail of the same coil, and coil against coil in the same batch
If gloss drifts, two side panels on one fridge read differently under bright light

Gloss and ΔE are a pair: one controls shine, the other controls hue. Both need batch samples on file.

White H-pattern prepainted sheet — White exposes gloss drift — beyond ±5 you can see it with the naked eye

All three tests run in our in-house lab, and batch data ships with the goods. If you need third-party salt spray or MEK reports (SGS/BV), say so when you order and we will arrange witnessed inspection.

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