Contents
Galvanised steel and aluminium are increasingly common in modern vehicle bodies and in industrial manufacturing. Yet when either metal is prepared out of habits formed on steel, adhesion and corrosion problems can follow; most of these stem from the metals' own surface chemistry and from small but critical mistakes made in the workshop. In this guide we look at the causes and the correct preparation steps together.
Why Is Adhesion Difficult?
Galvanised Steel: Zinc and White Rust
On galvanised steel, the steel is protected by the zinc layer on top of it. Zinc corrodes before the steel, effectively sacrificing itself to protect it; this mechanism is known as cathodic protection. Zinc, however, is a reactive metal. When it comes into contact with moisture and carbon dioxide, a loose layer of zinc oxide, hydroxide and carbonate, known as white rust, forms on its surface. Filler or primer applied over this layer bonds not to the metal but to this weak layer. Passivation residues and oils on new galvanised surfaces also make adhesion more difficult.
Aluminium: A Rapidly Re-forming Oxide Layer
As soon as aluminium comes into contact with air, it forms a thin, hard layer of aluminium oxide on its surface. Although this layer protects the metal, its chemically inert nature makes it difficult for paint and filler to adhere. On a fresh surface exposed by sanding, the oxide immediately starts to re-form and thickens over time. With aluminium, therefore, timing is at least as important as sanding.
Preparing Galvanised Surfaces
The zinc layer on automotive panels is mostly around 10 µm or thinner and can be removed completely in a few passes with a coarse abrasive. On hot-dip galvanised industrial parts the layer is thicker, but the basic principle is the same: create a key for adhesion while preserving the zinc. Large surfaces on industrial parts can also be roughened by light (sweep) blasting, provided that it does not thin the zinc layer more than necessary.
- Pre-cleaning: Before sanding, clean the surface of grease, silicone and dirt with ZN-530 Surface Cleaning Thinner using the two-cloth method.
- Remove white rust: If there is any white rust, remove it completely by mechanical means; never apply any product over this layer.
- Controlled sanding: Abrade the surface evenly, typically with P120–P180 or a fine abrasive pad. The aim is not to remove the zinc but to create a key for adhesion. Any spots where the zinc has been removed completely should be treated as bare steel and sealed with a suitable primer that provides corrosion protection.
- Final cleaning: Remove the dust and clean the surface again.
- Filler: If filling is needed, use ZN-170 Galva Filler, which is formulated for galvanised panels; standard polyester fillers may not achieve adequate adhesion on zinc. Once the filler has been dry-sanded, seal the area with a suitable primer.
Preparing Aluminium and Getting the Timing Right
- Pre-cleaning: Clean the surface with the two-cloth method before sanding.
- Sanding: Typically use P80–P120 in the area to be filled, and P180–P240 or a fine abrasive pad in areas that will only be primed. Aluminium is a soft metal, and an abrasive coarser than necessary leaves deep scratches. Because aluminium clogs abrasives faster than steel does, change them frequently and avoid excessive pressure.
- Final cleaning: Remove the dust, clean the surface again and do not touch it with bare hands.
- Apply without delay: Apply filler or primer as soon as possible after sanding. In practice, the aim is not to leave bare aluminium exposed for more than an hour. If the surface has been left exposed for a long time, repeat the light sanding and cleaning.
- Filler: If filling is needed, use a filler approved for aluminium substrates. On parts exposed to heat, such as an aluminium bonnet, ZN-160 Aluminium Filler is a strong option; confirm the conditions for application on aluminium in the product's Technical Data Sheet (TDS), and apply the first layer thinly, pressing down firmly with the spreader.
Galvanic Corrosion: Separate Tools and Abrasives for Aluminium
When aluminium and steel are in contact in a damp environment, galvanic (contact) corrosion occurs. In the workshop, the most insidious way this happens is steel dust becoming embedded in the aluminium surface. A sanding disc, wire brush or file previously used on steel carries microscopic steel particles onto the aluminium. Under the influence of moisture and salt, the aluminium around these particles corrodes, and months later the problem appears as bubbling and flaking under the paint. The same risk applies between an aluminium part and steel fasteners; on reassembly, the insulating elements and coated fasteners specified by the vehicle manufacturer must be refitted.
- Use separate sanding discs, backing pads, abrasive pads and hand tools for aluminium.
- If a wire brush is needed, use a stainless steel brush that is reserved for aluminium.
- If possible, carry out aluminium repairs in an area separate from steel work, and keep workbenches free of steel dust.
- Aluminium dust is flammable and, under certain conditions, can form explosive mixtures. Make sure it never comes together with sparks from steel grinding, use suitable dust extraction equipment and follow the warnings in the Safety Data Sheets (SDS) of the products you use.
Tip: Mark the abrasives and tools you reserve for aluminium with coloured tape and store them in a separate cabinet. A simple colour code is one of the most effective ways to prevent steel dust contamination when the workshop is busy.
Choosing the Right Filler and Primer
| Stage | Galvanised steel | Aluminium |
|---|---|---|
| Pre-cleaning and final cleaning | Two-cloth method | Two-cloth method |
| Sanding (typical) | P120–P180, without cutting through the zinc | Filler area P80–P120, primer area P180–P240 |
| Main risk | Removal of the zinc, white rust | Oxide re-forming, galvanic corrosion |
| Filler | Filler formulated for galvanised steel | Filler approved for aluminium substrates |
| Primer | 2K primer approved for the substrate | 2K primer approved for the substrate |
| Timing | Without delay after cleaning | As soon as possible after sanding |
When choosing a primer, the deciding factor is that the product is approved for the substrate in question. ZN-250 Joker Multi-Purpose Primer is a versatile 2K primer developed for use on a variety of substrates; confirm in the TDS the conditions for direct application on galvanised steel and aluminium, and whether a pre-primer is required. Remember, too, that polyester filler must not be applied over acid etch (wash) primer: the filler goes directly onto the prepared metal, and the primer follows the filler.
Common Mistakes
- Working galvanised steel with coarse abrasives as if it were ordinary steel and removing the zinc layer completely
- Cleaning only after sanding; if pre-cleaning is skipped, grease and silicone are ground into the surface during sanding
- Applying filler or primer without removing white rust
- Leaving sanded aluminium exposed for hours, or even until the next day
- Using the same discs, brushes and files on aluminium as on steel
- Applying standard polyester filler directly onto zinc
Summary
With galvanised steel, the goal is to create a key for adhesion while preserving the zinc layer; with aluminium, it is to seal the surface before the oxide re-forms and before steel dust can contaminate it. Correct cleaning, controlled sanding, a filler and primer suited to the substrate, and the right timing turn these two demanding metals into reliable substrates for paint. For special alloys or industrial parts, please get in touch with our technical team.