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Blistering in the paintwork on aluminium
What causes blistering — small bubbles — in the paint on aluminium, for example on a yacht’s aluminium mast or hull, and how can it be prevented?
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Summary:
Blisters in the paint on an aluminium yacht are almost always caused by corrosion developing beneath the coating, typically due to poor insulation, inadequate surface preparation, or moisture penetrating the paint system. Aluminium oxidises, expands, and pushes the coating away from the metal. Preventing this requires eliminating the source of the corrosion and rebuilding the coating system correctly using suitable primers and proper isolation.
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The most common causes
1. Filiform corrosion (hair-like threads beneath the paint)
This is the most common cause.
It occurs when moisture and oxygen get underneath the coating, often through:
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Poorly adhering primer
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Damage or scratches in the coating
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Inadequate degreasing or surface preparation
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Saltwater exposure, which accelerates the process
The result is small blisters and “worm-like” filaments under the paint.
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2. Galvanic corrosion beneath fittings
Stainless-steel hardware mounted on aluminium creates galvanic interaction.
Under the paint you’ll see:
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Chalky white oxidation
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Local expansion of the aluminium (oxide takes up more volume)
→ This lifts the coating and causes blistering.
Typical trouble spots include:
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Stanchion bases
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Mast and boom fittings
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Bolt holes
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Areas around windlasses and stainless deck hardware
3. Poor surface preparation of aluminium
Aluminium must be chemically and/or mechanically prepared before painting.
Common mistakes include:
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Inadequate degreasing
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No etch primer used
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No epoxy barrier coat applied
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Too much time between sanding, cleaning, and coating (aluminium oxidises immediately)
When paint is applied over fresh oxide, adhesion is poor and blistering is almost guaranteed.
4. Water ingress through sealant seams or bolt holes
Even a small leak can draw moisture between the coating and the metal.
This leads to:
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Localised oxidation
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Paint lifting or swelling
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Plaster-like blisters along seams and joints
5. Paint systems not suitable for aluminium
Using the wrong coating (e.g., polyurethane without a proper primer system) will inevitably result in failure.
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What you can do about it
1. Eliminate the source of the problem
Where blisters occur around stainless-steel fittings, the metals must be isolated using:
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Duralac
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Tef-Gel
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Nylon or plastic washers
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Rubber or plastic shims
→ This stops galvanic corrosion from developing.
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2. Expose and treat the affected area
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Strip all coatings back to clean, bare aluminium
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Mechanically abrade and chemically clean the surface
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Apply an aluminium etch wash if required (Alodine/chromate or a chromate-free equivalent)
3. Rebuild the coating system correctly
A reliable paint system typically consists of:
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Thorough degreasing (acetone or a dedicated aluminium cleaner)
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Etch primer
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Several coats of epoxy primer
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PU topcoat or another two-part finish
→ The epoxy barrier coat prevents moisture ingress.
4. Improve ventilation (interior surfaces)
For blistering on the inside of the hull or superstructure:
Condensation → Corrosion → Coating failure.