Degreasing before powder coating

A powder coat that blisters along a hem eighteen months after installation usually had oil under it from the day it was sprayed. Degreasing before powder coating is the step that decides adhesion, and it is the step buyers most often compress into two words on a drawing: clean and degrease.

The problem is that contamination is invisible and the failure is slow. A part wiped with a rag looks ready and passes a visual check at the booth. Six months later the film lifts at an edge, water gets behind it, and the whole panel creeps. By then nobody can tell whether the oil came from the press, the laser, the welder's gloves or the forklift.

What does degreasing before powder coating have to remove?

Five kinds of soil arrive on a fabricated steel part, and each needs a different part of the cleaning cycle.

Stamping and drawing lubricant is the heaviest. A drawn or deep-formed part can carry a film thick enough to feel, and it sits in the grain of the surface as well as on top of it.

Cutting and machining residue from laser cutting, drilling and tapping includes fine metal particles held in oil. These particles survive rinsing and show up as roughness under the powder.

Rust preventive is often applied between operations and is designed to resist washing off, which makes it the hardest film to remove.

Handling contamination comes from gloves, skin oils and sweat. Salts from skin are the reason a fingerprint becomes a blister.

Weld and heat scale on heavier sections is not oil, but it behaves the same way in the booth: it stops the conversion coat from forming and the powder keys to the scale instead of the steel.

To put the quantity in perspective, a 1 µm film of oil over one square metre weighs about 0.9 g. That is roughly the mass of two paper clips spread across a panel, and it is more than enough to drop adhesion from a pass to a fail.

Why does a clean-looking part fail the water-break test?

Because the eye reads gloss, not surface energy. A part with a thin oil film looks better than a properly cleaned one, since the oil fills the micro-texture.

The water-break test is the cheapest check in the paint shop. Pour demineralised water over a vertical surface and watch it for 30 seconds. Clean steel wets evenly and holds a continuous film, while contaminated steel pulls the water into beads and leaves a visible break pattern. A part that beads goes back to degreasing; it does not go to the booth.

What are the seven tanks actually doing?

A seven-tank line separates the jobs that a single cleaning tank cannot do at once: removing oil, removing oxides, conditioning the surface and rinsing off every chemical it applied. The order matters as much as the chemistry.

The gap between tank 1 and tank 7 is where most failures start. A cleaner that is past its life leaves a film. A rinse that carries drag-out from the previous tank deposits the very chemistry it was meant to remove. A final rinse with hard water leaves calcium salts that expand under the film in humid service.

How do you know the bath is working?

Bath control is a schedule, not an opinion. Four measurements catch nearly every drift.

• Free and total alkalinity by titration, which tells you whether the degreaser still has capacity to saponify oil.
• Conductivity and pH of each rinse, which show how much drag-out is accumulating and when to dump and refill.
• Temperature, because a bath at 40 °C when the process sheet says 55 °C cleans at roughly half the rate.
• Coating weight of the phosphate, which shows whether the conversion coat is forming or just staining the surface.

Add a visual standard for the cleaned part, agreed with the customer at first article. A photograph of an acceptable and a rejected phosphate colour is more useful to a night shift than a text description.

Does shot blasting replace degreasing?

No. Blasting and degreasing remove different things, and a blasted part still carries oil.

Shot blasting removes mill scale, rust and old coating, and it leaves a rougher surface that helps adhesion. It does not remove drawing lubricant sitting in the pits it creates. In fact, blasting can drive oil and blasting dust into the surface, which is why a blasted part goes through the same alkaline degrease and rinse sequence afterwards.

The order matters. Blast first to remove scale and rust, then degrease, then phosphate. A part blasted and sent straight to the booth has a clean-looking, uniformly grey surface with a film of oil on it, and it fails the water-break test every time.

How does the cleaner change for aluminium?

Aluminium needs a different first tank from steel. A strong alkaline cleaner attacks the aluminium itself, etching the surface and leaving a smut that no rinse will remove. A milder, silicate-buffered cleaner holds the pH in a range that removes oil without eating the metal.

After cleaning, aluminium usually needs a deoxidising step to remove the natural oxide and the smut, and then a chrome-free conversion coat rather than the iron phosphate used on steel. The conversion coat on aluminium is thin, often 0.1–0.5 g/m², so it is checked by colour and by adhesion rather than by coating weight alone.

Mixing aluminium and steel parts on the same rack creates a galvanic couple in every rinse tank. Separate loads, or separate racks, and tell the paint shop which substrate is coming.

What film thickness and cure should the specification ask for?

Film thickness depends on the service environment, but the pretreatment underneath is the constant.

For indoor electrical equipment and rack parts, 60–90 µm of polyester powder over a phosphate coat is the normal range. Outdoor cabinets and genset canopies move to 80–120 µm. Coastal and heavy industrial sites need the same film over a better conversion coat, and often a primer, because extra powder on a weak interface just gives a thicker layer to peel.

Cure is measured on the metal, not in the oven air. Powder typically needs 180–200 °C part temperature for 10–15 minutes, and a heavy 6 mm section takes much longer to reach that than a 1 mm panel sharing the same rack. An undercured film looks correct, measures correct on DFT and fails an impact test.

Acceptance usually includes DFT by magnetic gauge on a flat face and on an edge, adhesion by cross-hatch to a stated class, a cure check by solvent rub, and a visual check under agreed light. Ganesh runs DFT measurement and cross-hatch adhesion testing in-house on the paint shop route.

What salt spray test should I ask for?

Salt spray is the number buyers quote to each other, and it is easy to specify in a way that proves nothing. ISO 9227 neutral salt spray at 240, 480 or 720 hours is the common range, but the result depends on the substrate, the conversion coat, the film thickness and whether the panel is scribed.

Three questions make the requirement meaningful. Is the test on a flat panel or on a production part with edges and a weld? Is there a scribe, and what creep is allowed from it? And which defects fail: blisters, colour change, base metal corrosion or adhesion loss?

A 720-hour pass on a flat, correctly pretreated panel says little about a hem that was never degreased. If the service environment is the real concern, tie the test panel to a production-representative part. The outdoor coating failure guide covers what the salt spray misses.

Which failures trace back to degreasing?

Four defect patterns account for most warranty claims, and all four point upstream of the booth.

Blistering in a cluster or along a line follows a contaminated area: oil, a fingerprint, or rinse salts. The blister contains liquid or gas that came from under the film.

Pinholes and craters over a weld or a hem mean trapped oil or cleaning fluid boiled out during cure. They appear in the same place on every part, which is the clue.

Adhesion loss on a whole panel, with the coating peeling in sheets, usually means the conversion coat did not form. That can be a bath problem or a surface that never got wet properly.

Rust creep from an edge is the edge-coverage problem: too little film on a sharp corner, or a burr that shadowed the spray. Deburring before pretreatment is part of the coating specification.

What Ganesh runs for pretreatment and coating

Ganesh runs seven-tank pretreatment in-house ahead of an 18 ft × 14 ft electrostatic powder booth, with a wet paint booth and a curing oven on the same route. Heavier parts are cleaned in a 16 ft × 16 ft shot blast chamber before pretreatment where scale or heavy rust calls for it, and the effluent treatment plant handles the process water under ISO 14001:2015. The plants hold ISO 9001:2015 and ISO 14001:2015, and the coating route supplies enclosure, canopy and elevator programmes for customers including Mahindra Powerol, Godrej & Boyce and TKE.

Film thickness is measured with a DFT gauge and adhesion checked by cross-hatch on the coated part, so a batch is released on evidence rather than appearance. The paint shop capability page sets out the tanks, booths and ovens, and the surface treatment hub holds the tank-by-tank guides.

What the finish specification has to settle

• Substrate, including whether any part is galvanised or zinc coated.
• Cleaning requirement: alkaline degrease only, or degrease plus phosphate.
• Coating system, colour reference and gloss level.
• Target DFT range, with the measurement points and the minimum on edges.
• Cure requirement, expressed as metal temperature and time.
• Masking: threads, earth studs, mating faces, sliding surfaces and any area that must conduct.
• Corrosion test, panel type and acceptance criteria if a salt spray result is required.
• Handling and packing, because a coated panel damaged in transit is a reject at the customer, not at the plant.

Send the specification through the quotation route with the substrate and the service environment. For an early cost view, the powder coating cost estimator shows how part size, film thickness and volume change the finishing cost, and the seven-tank pretreatment process guide explains each stage in more detail.

When powder coating is the wrong finish

Powder is a poor choice where the part will be handled heavily in service, because a thick film chips at impact and the chip is hard to touch up on site. Wet paint or a powder primer plus topcoat is better for a large visible surface with a deep colour requirement.

It is also the wrong finish over a substrate that needs a different chemistry. Galvanised steel, aluminium and stainless each need their own pretreatment, and running them through a mild steel cycle produces exactly the adhesion failure the cleaning was meant to prevent. If the part is zinc coated, say so on the drawing before the process sheet is written.

Finally, part size has a limit. An 18 ft × 14 ft booth sets the maximum envelope, and anything longer needs a split, a different booth or a field-applied finish. Ask before the design is fixed, not after the first article. The companion guide on diagnosing pretreatment faults covers what to check when a batch fails.