Coating Delamination and Poor Adhesion: Root Causes on the Line

Provided by Lin Cong, Technical Director, MUSI Technology · Published October 22, 2026 · Last reviewed October 22, 2026

Quick answer: Most coating adhesion failures start before the coating booth. Oil left after degreasing, a conversion bath out of control, contaminated rinse water, water or flash rust left after dry-off, and soluble salts or dust after blasting cause the film to part at the metal. Under-cure, over-cure and an exceeded recoat window cause failures inside or between coats. The plane where the film separates tells you which stage to check first.

Where did the coating come off?

Before changing anything, peel or cut the failed area and look at the back of the flake and the surface it left. The failure plane narrows the search from the whole line to one or two stages.

Quantify the problem with a cross-cut or pull-off test before and after each change; the methods and classes are in our guide to coating quality tests.

What are the root causes on a coating line?

Why do most adhesion failures trace back to pretreatment?

Pretreatment removes oil, dust, rust and mill scale and deposits a thin conversion layer that anchors the coating and slows corrosion creep. If either job is incomplete, the best powder and a perfect oven cannot bond to the metal. The washer is also the stage that drifts quietly: a cleaner that loses concentration or a rinse that builds up salts still looks normal from the outside.

The chemical supplier’s technical data sheet for the product in the tank takes priority over these typical values. See phosphate vs zirconium pretreatment and iron vs zinc phosphate for bath control in more depth.

Which substrates need special attention?

Steel shot and grit must not be used on aluminium or stainless steel, because embedded iron particles cause corrosion spots later. Most powder systems want a blast profile of 25-75 microns; heavy-duty liquid systems want 50-100. See blasting media selection and pinholes and outgassing.

In what order should you investigate an adhesion failure?

1. Identify the failure plane on several failed parts and keep samples.
2. Run a cross-cut on parts from the current production to see whether the problem is still present.
3. Check the washer: titrate every bath, measure rinse conductivity, and do a water-break test after the final rinse.
4. Check dry-off: are parts leaving dry, with no water in pockets and no flash rust?
5. Run an oven datalogger with the probes on the heaviest and lightest parts on the rack.
6. For blasted steel, check profile, dust and soluble salts, and the time between blasting and coating.
7. Only then look at the powder or paint batch; ask the supplier for the batch certificate and a retained sample.
8. Change one variable at a time and re-test adhesion after each change.

Which components and test kits are commonly used?

Key components are available from international brands such as these, configurable per customer specification. Brand names are for orientation and do not imply a partnership.

How does MUSI design a line against adhesion failures?

MUSI Technology (Changzhou MUSI Environmental Technology Co., Ltd.) builds turnkey coating lines in Changzhou, China, and has delivered 800+ coating lines to 500+ clients in 36 countries. Adhesion depends on the washer, the dry-off oven, the booth and the curing oven working together, so one engineering team designs all of them — together with dust collection, VOC treatment and pretreatment wastewater treatment — under one contract.

• Washer stages sized for the part’s oil load, with conductivity and dosing control on the rinses.
• Dry-off and curing ovens sized from line speed and the coating maker’s cure schedule.
• Film build, cure profile and line speed written into the contract and demonstrated on your own parts at the factory trial run.

See the pretreatment washer and the powder coating curing oven, or send photos of the failed parts and your process data.

More buyer questions on cost, line selection, process and emissions are answered in Coating Line Questions: 100 Answers from MUSI Engineers.

Frequently asked questions

Why is my powder coating peeling off?

If the film comes off leaving clean metal, the cause is almost always pretreatment: oil not removed, a conversion bath out of control, contaminated rinse water or water left after dry-off. If coating remains on both surfaces, check cure with an oven datalogger. If a topcoat lifts from a primer, check the recoat window and the first coat’s cure.

Can under-cure cause poor adhesion?

Yes. An under-cured film has not fully cross-linked, so it is soft, fails a solvent rub and can fail cohesively or lose adhesion at the substrate. The cure clock starts only when the metal reaches the temperature on the powder data sheet, so check part temperature, not oven air.

How do you test whether a part is clean enough to coat?

Use a water-break test after the final rinse: a clean surface holds a continuous water film for about 30 seconds, while an oily one breaks into droplets. For blasted steel, add a dust tape test to ISO 8502-3 and a soluble salt test to ISO 8502-6 and 8502-9.

Why does a topcoat peel from the primer?

Usually because the primer was over-cured or the time between coats exceeded the recoat window on the data sheet, leaving a surface the topcoat cannot bond to. Contamination between coats is the other common cause. Partial-curing the primer where the data sheet allows, or abrading it before recoating, solves most cases.

Does galvanised steel need special pretreatment?

Yes. Zinc surfaces carry zinc salts and often a passivation layer, and they can outgas during cure. Sweep blasting or a zinc-specific conversion improves adhesion, and a pre-bake or an outgassing-forgiving powder prevents pinholes.

What rinse water quality is needed before coating?

Typical guidance is a final rinse no higher than 1.5 times source water TDS, a rinse after zirconium below 50 µS/cm with 25 µS/cm recommended, and a fresh DI or RO halo below 10 µS/cm at the stage exit. The chemical supplier’s data sheet takes priority.

About the figures in this guide. Bath and rinse values are typical figures from chemical suppliers’ data sheets and industry guidance, as published on MUSI’s pretreatment washer page; the data sheet for the product in your tank always takes priority. Standards are cited by number; work to the edition named in your specification. Brand names are listed for orientation only.