What to check on a Topside Paint.
Topsides sit above the boottop and below the main deck, exposed to sun, salt-laden air, rain and fendering contact at berth, but never to continuous immersion. The coating's job is different from an underwater hull or ballast tank system: barrier protection against corrosion still matters, but gloss retention, colour stability under UV and resistance to chalking are what the owner actually judges the paint job on. A topside system that holds a thinner, glossier topcoat will fail faster under mechanical abrasion than a boottop system, and a boottop-style heavy build…
What sets topside coatings apart
Topsides sit above the boottop and below the main deck, exposed to sun, salt-laden air, rain and fendering contact at berth, but never to continuous immersion. The coating's job is different from an underwater hull or ballast tank system: barrier protection against corrosion still matters, but gloss retention, colour stability under UV and resistance to chalking are what the owner actually judges the paint job on. A topside system that holds a thinner, glossier topcoat will fail faster under mechanical abrasion than a boottop system, and a boottop-style heavy build coat looks chalky and dull if used on topsides. The two are not interchangeable even though both sit above the waterline in places.
Coating build-up
Primer
Shop primer or a zinc-rich or epoxy anti-corrosive primer bonds to blasted or mechanically prepared steel and carries most of the barrier function. Surface preparation standard (typically Sa 2½ blast for newbuild, hand/power tool for maintenance) decides how long this coat lasts more than the primer chemistry itself.
Intermediate / build coat
A high-build epoxy or modified epoxy coat brings the system up to specified dry film thickness and evens out surface profile before the topcoat goes on. This is the layer most often skipped or thinned on a rushed repaint, and it is the layer that determines total system life.
Topcoat
Aliphatic polyurethane or acrylic topcoats give UV and gloss resistance. Two-pack polyurethanes hold gloss and colour longest but need tighter application control (temperature, humidity, pot life); one-pack systems are more forgiving for maintenance touch-up but chalk and fade sooner.
Selection and sizing
- Total dry film thickness (DFT) across the system, usually specified per coat and as a system minimum in microns
- Application temperature and humidity window, and the margin above dew point required for each product
- Recoat interval (minimum and maximum) between build coat and topcoat — miss the maximum and intercoat adhesion suffers
- Gloss level and colour, matched against the existing system if this is a partial repaint rather than a full newbuild specification
Regulations and class
There is no dedicated IMO performance standard for topside coatings comparable to the Performance Standard for Protective Coatings that governs ballast tanks and double-side skin spaces — topside specification is set by the owner and yard, not by a mandatory IMO instrument. Class societies do check that newbuild coating application records (surface preparation grade, DFT measurements, environmental conditions during application) are retained, since these underpin the coating warranty rather than a statutory requirement. Port state control has no specific topside coating checklist, though visible extensive corrosion or delamination can trigger a wider structural inspection.
Typical faults
| Cause | Consequence |
|---|---|
| Recoat window exceeded between build coat and topcoat | Intercoat adhesion failure, topcoat peels in sheets rather than wearing evenly |
| Application below dew point margin | Moisture entrapment, blistering appearing months after the repaint |
| Thinned build coat to save material | Early substrate corrosion showing through as rust staining under an intact-looking topcoat |
| Prolonged UV exposure without maintenance washdown | Chalking, colour fade, and loss of gloss well before the coating's stated service life |
What to look for in a supplier
- Data sheets stating DFT ranges, recoat windows and application limits, not just headline durability claims
- Documented compatibility with the existing coating generation if this is a maintenance repaint over an unknown history
- Consistent colour batch-to-batch, since topside repairs are the most visible coating work an owner will judge
- Field technical support for surface preparation inspection during application, not only a product data sheet
A topside repaint that looks perfect on delivery day and blisters within a season almost always traces back to application conditions, not the product — check the dew point and recoat interval log before blaming the paint.