A paint system's life depends less on the paint than on surface preparation and application conditions. In a build, painting advances through check points approved before each coat.
Surface preparation
- Steel
- Blast cleaning grade (ISO 8501-1) and surface profile; the time from blasting to primer is kept short.
- Aluminium
- Suitable pretreatment and primer; no copper-based products.
- GRP
- Sanding and cleaning on gelcoat; primer and filler in a painted system.
Application conditions
- Dew point: surface temperature at least 3 °C above the dew point; otherwise invisible moisture is trapped under the paint.
- Relative humidity and temperature: within the paint maker's limits.
- Overcoating time: minimum and maximum; if exceeded, sanding between coats is needed.
Check points
- Preparation sign-off. Cleanliness grade and profile.
- Before each coat. Ambient values logged.
- Dry film thickness. Measured on every coat by the yard or paint supplier and recorded.
- Fairing and filler. Fairness checked with light; filler thickness within the maker's limit.
- Topcoat. Appearance, colour and gloss.
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- Next stage: interior: Materials, access and checks.
Checking the preparation grade
Surface preparation, more than the paint itself, sets how long a coating lasts. On steel, the surface is blasted to the cleanliness grade the paint system requires and primed soon after; bare blasted steel rusts quickly. On GRP and aluminium, the surface is sanded and cleaned of dust and grease. The grade achieved is compared with the value in the paint maker's data sheet.
Dew point
Paint must go on when there is no condensation on the surface. So air temperature, relative humidity and surface temperature are measured and the dew point worked out. Common practice is for the surface to be at least a few degrees above the dew point. These readings are taken every painting day and recorded.
Film thickness
Each coat's thickness is measured with a wet film comb during application and a dry film gauge after drying. Too thin a coat weakens protection; too thick can cause cracking and drying problems. Readings are recorded and compared with the target in the paint system.
Overcoating times
How long to wait between coats is stated in the data sheet according to temperature. Ignoring it can lead to poor adhesion between coats.
Final coat and acceptance
The final coat's appearance is checked in good light and from different angles for runs, orange peel and dust. Acceptance rests on the readings and the visual check together.
Records
The paint system used, batch numbers and readings go into the handover file, so later maintenance painting uses compatible products.
Underwater coatings
Below the waterline, the barrier coat and antifouling are part of the same system and are checked the same way, coat by coat.
Painting outdoors
If painting is done in the open rather than in a shed, wind, dust, sun and overnight condensation add risk. Application is then timed for after the surface has dried from morning dew and before evening condensation, and the area is shielded from wind and dust. Days lost to weather relate to the weather allowance in the contract.
Outdoors, readings are taken more often and logged daily. If conditions are unsuitable, painting is postponed despite schedule pressure; a coat applied in poor conditions may have to be stripped and redone later, which costs far more than the lost day.
Patience here pays back for years.
Log each day's readings.