Powder Coating Quality:
What to Specify
and How to Verify

The Pre-Treatment Stage Is the Most Important

Powder coat adhesion depends almost entirely on the quality of surface preparation before the powder is applied. The coating itself is straightforward — the pre-treatment line is where quality is won or lost.

A proper pre-treatment sequence for SPCC steel includes at minimum:

Vu Trinh operates a 5-stage pre-treatment line covering all of the above steps. Single-stage or 3-stage lines — common at lower-cost fabricators — skip the phosphate step and produce coating that will delaminate under load or in humid conditions.

Film Thickness: What to Specify

Powder coat film thickness (DFT — Dry Film Thickness) directly affects both appearance and corrosion protection. Too thin and coverage is poor; too thick and it adds unnecessary weight and cost, and risks texture problems.

Application	Typical DFT Specification	Notes
General indoor (enclosures, panels)	60 – 80 μm	Standard industrial specification
Outdoor / weathering applications	80 – 120 μm	Increased film for UV and moisture resistance
Decorative / consumer-facing	60 – 100 μm	Texture finish typically 80–100 μm
Heavy industrial / chemical exposure	120 – 200 μm	May require 2-coat system

When specifying, state both minimum and maximum values. A drawing note of "Powder coat: RAL 7016, DFT 60–80 μm" is sufficient for most industrial parts.

Adhesion Testing: ISO 2409 Cross-Cut Test

The most practical on-site adhesion test for powder coat is the cross-cut test (ISO 2409 / ASTM D3359). It requires only a cutting tool, a template, adhesive tape, and a magnifying glass — no expensive equipment.

Specify "ISO 2409 cross-cut adhesion: Class 0 or Class 1" on drawings where coating adhesion is critical. For general industrial parts, Class 1 is the standard acceptance criterion.

Colour Specification

Always specify colour using a recognised system rather than a description. The two most common systems in industrial sheet metal:

• RAL Classic — widely used, good availability in Vietnam. Example: RAL 9005 (jet black), RAL 7016 (anthracite grey), RAL 3000 (flame red)
• Pantone — used when brand colour accuracy is critical. Note that Pantone colours must be matched by the powder supplier and may require custom mixing

Specify gloss level alongside colour: Matte (10–30 GU), Satin (30–60 GU), or Gloss (60–90 GU) measured at 60°. If left unspecified, fabricators will use whatever is in stock — typically satin.

Common Powder Coating Defects and Their Causes

Defect	Appearance	Root Cause
Fish eyes / craters	Small circular depressions in the film	Oil contamination during pre-treatment — degreasing failure
Pinholes	Tiny holes through to substrate	Moisture on part surface entering oven, outgassing from steel
Orange peel	Textured surface resembling orange skin	Film too thick, incorrect oven temperature, or incompatible powder
Delamination / peeling	Coating lifts from surface	Missing or inadequate phosphate conversion step
Colour variation between batches	Visible shade difference	Different powder batches, inconsistent cure temperature
Thin coverage at edges/corners	Visible steel at sharp edges	Faraday cage effect — requires edge rounding in design or dedicated spray pass

What to Include on Your Drawing

A complete powder coat specification on a technical drawing should include:

• Colour reference (RAL or Pantone number)
• Gloss level (matte / satin / gloss or GU value)
• DFT range (e.g. 60–80 μm)
• Adhesion test requirement (e.g. ISO 2409 Class 1)
• Any areas to mask (threaded holes, mating surfaces, ground points)

Example drawing note: "Finish: Powder coat RAL 9005 matte (10–20 GU), DFT 60–80 μm, ISO 2409 Class 1 minimum. Mask: M4 threaded inserts."