Preparing Steel, Aluminum, and Galvanized Surfaces

First, Identify What You Are Coating

The preparation process should never begin with a guess. Similar-looking metals may react differently to cleaners, acids, alkaline materials, abrasives, and conversion treatments.

Before processing the part, confirm:

• The substrate type.
• The alloy or grade when it affects the process.
• Whether the metal is bare, plated, galvanized, or previously coated.
• The type of oil, lubricant, drawing compound, or fabrication soil present.
• Whether the part contains mixed metals.
• The required service environment.
• The customer’s pretreatment and coating specification.

Mixed-metal assemblies deserve particular attention. A cleaner or pretreatment suitable for one metal may attack another. Different metals can also respond differently during rinsing, drying, grounding, heating, and corrosion exposure.

Preparing Carbon Steel

Carbon steel is strong, widely available, and easy to fabricate. It is also vulnerable to corrosion. Moisture and oxygen can begin creating rust before the part ever reaches the finishing line.

Common surface conditions include:

• Machining and stamping oils.
• Grease and shop dirt.
• Rust and flash rust.
• Hot-rolled mill scale.
• Welding smoke and heat discoloration.
• Weld spatter and anti-spatter compounds.
• Laser-cut edge oxidation.
• Grinding dust and abrasive residue.
• Fingerprints and handling contamination.

Cleaning Steel

Oils and manufacturing soils must be removed using a cleaning process appropriate for the soil and substrate. Cleaning may be performed by spray washing, immersion, hand wiping, vapor or solvent methods where permitted, or another approved process.

The cleaner must reach the contamination. Overlapping joints, tubes, channels, deep recesses, and tightly packed racks can prevent complete contact, rinsing, and drainage.

Removing Rust and Mill Scale

Rust and mill scale are not dependable foundations for a powder coating. They may require abrasive blasting, mechanical cleaning, chemical treatment, or a combination of methods.

Mill scale may appear tightly attached, but if it later separates from the underlying steel, the powder coating separates with it. The coating may remain bonded to the scale while the scale is no longer bonded to the part.

Abrasive blasting can remove scale and corrosion while creating a surface profile. That profile must be appropriate for the specified coating thickness. Excessively deep profile peaks may not receive enough coating coverage.

Pretreating Steel

Chemical pretreatments for steel may include iron-phosphate, zinc-phosphate, zirconium-based, silane-based, or other approved technologies. The selection depends on the required corrosion resistance, equipment, chemistry supplier, water quality, substrate, and customer specification.

A light-duty indoor product and an exterior product in a severe corrosive environment may require very different systems. There is no universal pretreatment that provides the same performance on every steel part.

Preparing Aluminum

Aluminum does not rust like carbon steel, but it does oxidize. A thin oxide film naturally forms on its surface. The metal may also carry rolling oils, lubricants, polishing compounds, smut, storage stains, fingerprints, or corrosion products.

Aluminum alloys do not all behave identically. Castings, extrusions, sheet, and machined aluminum may require different cleaning or pretreatment adjustments.

Cleaning Aluminum

The cleaner must remove oils and soils without causing uncontrolled attack, staining, or excessive etching. A cleaner designed for steel may be too aggressive for aluminum.

Process control is important. Concentration, temperature, contact time, pH, and contamination can influence how the aluminum surface responds.

Etching and Deoxidizing

Depending on the substrate and process, aluminum may be etched, deoxidized, desmutted, or otherwise conditioned before pretreatment. These stages remove or modify unwanted oxide and surface residue.

More etching is not automatically better. Excessive metal removal can change dimensions, increase chemical consumption, create surface roughness, or make rinsing more difficult.

Pretreating Aluminum

Aluminum pretreatment may use approved chrome-free conversion coatings or other systems specified for the application. Architectural aluminum may require compliance with recognized performance programs and specifications.

The complete system must be qualified:

• Aluminum alloy and surface condition.
• Cleaning and etching process.
• Rinse-water quality.
• Conversion treatment.
• Powder chemistry.
• Film thickness.
• Cure schedule.
• Required corrosion and weathering performance.

A coating labeled for exterior use does not automatically satisfy an architectural specification. The pretreatment, powder, applicator, process, and test requirements may all be part of the approval.

Preparing Galvanized Steel

Galvanized steel is steel protected by a zinc coating. The zinc provides corrosion protection, but it also creates preparation concerns that differ from ordinary carbon steel.

The surface may contain:

• Oil or fabrication residue.
• Passivation or temporary protective treatments.
• White rust or other zinc corrosion products.
• Storage stains.
• Surface irregularities from the galvanizing process.
• Trapped gases or moisture.
• Sharp points, runs, drips, or rough areas.

New Galvanized Steel Is Not Automatically Ready

A new galvanized surface may look bright and clean while carrying oils, passivation, or treatments that interfere with coating adhesion. Verify the galvanizer’s process and any post-treatment before selecting the preparation method.

Removing White Rust and Contamination

White rust is a zinc corrosion product that can form when moisture remains trapped against the galvanized surface. It must be evaluated and removed using a method suitable for the condition and coating specification.

Cleaning must remove oil and contamination without unnecessarily damaging the protective zinc layer.

Sweep Blasting

Sweep blasting may be used to clean and lightly roughen galvanized surfaces. It is a controlled, lower-intensity operation—not the same as aggressively blasting heavy mill scale from carbon steel.

Excessive pressure, poor abrasive selection, incorrect angle, or prolonged blasting can remove or damage the zinc coating. The blasting procedure should be qualified and performed by trained personnel.

Chemical Pretreatment

Galvanized surfaces may also receive a compatible chemical pretreatment. The process must be approved for zinc surfaces and controlled according to the chemical supplier’s instructions.

The chemical system must account for the type and condition of the galvanized layer. Hot-dip galvanized parts, electrogalvanized sheet, zinc-rich surfaces, and fabricated galvanized assemblies may not respond identically.

Outgassing Requires Special Attention

Cast aluminum, cast iron, galvanized steel, porous metal, and certain welded assemblies can release gases when heated. Oils, moisture, air, or process residue trapped in pores, seams, and surface irregularities may escape while the powder is melting.

The result may include:

• Pinholes.
• Bubbles.
• Craters.
• Blisters.
• Rough or uneven film.

Possible control methods may include preheating or prebaking the part, modifying part design, improving cleaning and drainage, changing the heating rate, or using an outgassing-forgiving powder. The correct response depends on the source of the gas.

Fabrication Can Help—or Hurt—the Process

The preparation department should not be expected to repair every fabrication problem. Part design and fabrication affect cleaning, rinsing, drying, grounding, coating, and cure.

Inspect parts for:

• Sharp edges and burrs.
• Weld spatter and rough welds.
• Anti-spatter compounds.
• Overlapping seams that trap oil or solution.
• Blind holes and cavities that retain rinse water.
• Tubing without adequate drain and vent holes.
• Dissimilar metals joined in one assembly.
• Heavy and thin sections that heat at different rates.
• Areas difficult to ground or electrostatically coat.

Sharp edges deserve particular attention. Powder can pull away from an edge while melting, producing less protection at the exact location most vulnerable to corrosion and damage.

Rinsing Is Part of Surface Preparation

Cleaning and pretreatment chemicals must be rinsed as required by the process. Poor rinsing can leave residue that interferes with adhesion, appearance, or corrosion resistance.

Rinse performance depends on:

• Water quality.
• Rinse-stage cleanliness.
• Overflow and replacement rate.
• Spray pressure and nozzle condition.
• Part spacing and orientation.
• Contact time.
• Drainage between stages.
• Final-rinse requirements.

Dragging contaminated solution from one stage into the next can gradually upset the entire process. Rinse stages should be monitored rather than treated as tanks containing “just water.”

Drying Must Reach the Difficult Areas

Prepared parts must be completely dry before powder application. Water can remain in seams, threaded holes, channels, cavities, porous castings, tubing, and closely fitted joints.

The dry-off process must be validated using the most difficult production parts. A flat test panel may dry quickly while a heavy fabricated assembly carries water into the powder booth.

Part orientation should allow solutions to drain. The dry-off oven should remove moisture without damaging the pretreatment or causing new contamination.

Prevent Recontamination

Once a surface has been prepared, it must be protected. Bare hands, dirty gloves, hooks, carts, grinding dust, welding smoke, oil mist, and condensation can undo good preparation.

Establish controls for:

• Clean gloves and part handling.
• Hook and rack cleanliness.
• Maximum delay before coating.
• Protection from nearby fabrication and maintenance work.
• Indoor storage of prepared parts.
• Temperature changes that may cause condensation.
• Reinspection after delays or interruptions.

If rust, oxidation, moisture, dust, or handling contamination appears after preparation, the part should not simply be sent through the powder booth. It must be evaluated and corrected.

Comparison of Preparation Concerns