July 24, 2026 | Tech Notes
Prepainted steel has two destinies: on a roof, or inside an appliance. Both look like galvanized substrate plus a coat of paint — but if you buy on price per tonne alone, you will probably receive construction grade, and find out on your stamping line. The difference is not in the brochure. It is in eight numbers you can write into a contract.
| Item | Construction grade (roofing, cladding) | Appliance grade (fridges, washers) |
|---|---|---|
| Substrate grade | SGCC / S250GD commercial | DX52D+Z / DX54D+Z stamping |
| Zinc coating | Z60–Z120 | Z100–Z180, higher for doors |
| Topcoat thickness | 15–20 μm | 20–25 μm, measured every batch |
| Back coat | None or thin primer | 5–8 μm epoxy, against condensation rust |
| Gloss (60°) | 30–70 GU, ±10 | 20–60 GU, ±5 recorded per batch |
| Salt spray | 200–300 h | 500 h and up |
| MEK rubs | >50 to substrate | >100 to substrate |
| Color ΔE | Not batch-controlled | Batch samples kept, ΔE logged |
Appliance panels get stamped and folded: the paint must survive T-bends and cupping tests, which is what topcoat thickness and cure are for. The 100 MEK rubs simply measure whether the film was baked through. Construction sheet never sees a press, so nobody checks.
Washing machine and fridge panels live next to humid air for years. Without an epoxy back coat, condensation creeps under the film from cut edges and the back, and blisters show up in year two. A construction panel faces a wall — nobody cares. Gloss and ΔE control whether a month of production batches look like one color on the same assembly line. One building can be one color; a fridge line cannot.
If the answers are vague, you are talking to a construction-grade mill with a new business card.
Every batch leaving our plant carries film thickness, gloss and color data, and salt-spray / MEK reports can ship with the goods. Send your drawing and target numbers — we make a sample first, then book the run.
