Choosing and applying antifouling paint

Choosing and applying antifouling paint

While a ship’s hull spends most of its time underwater and out of site, it must remain very much in mind in terms of its protective coating.

That’s because ships are at risk of fouling—the attachment to a submerged surface of sea organisms like grass or barnacles. Fouling can cause a few different problems that begin to snowball if they’re not quickly addressed, including:

  • Adding significant drag, slowing vessels in transit
  • Added drag means ships must work harder —and expend more fuel— to maintain speed
  • Fouling on a hull left unchecked can also spur corrosion, adding another element to the problem

In Painting Manual Vol.1: Good Painting Practice, it is estimated that the marine industry could save between 37 and 75 million tons of fuel each year if proper antifouling measures are taken on the 80,000-plus seafaring vessels in operation.

Choosing the right antifouling coating

Antifouling coatings come in conventional, long life, self-polishing, and fouling release formulations. Consider the characteristics of each:

Conventional formulations are also known as soluble matrix or ablative coatings. Resin, which dissolves in seawater, is used as the binder. Biocides mixed in with the resin leach into the seawater as the resin degrades, preventing growth of fouling organisms. The effectiveness of conventional antifouling coatings wanes over time; they usually last around a year before replacement is recommended.

Long-life antifoulings are formulated in insoluble matrices. Biocide leaches out of the paint film, leaving a porous skeleton-like layer behind. The release rate of the biocide decreases as that porous layer builds. These systems can last up to two years; the porous layer must be removed prior to re-coating for best results.

Self-polishing coatings are essentially high-performance ablative systems that degrade in seawater. However, these hydrolysable formulations include additional binders that react with seawater to promote a more stable degradation. That, in turn, creates a smoother hull surface. Self-polishing antifoulings can last up to five years.

Fouling release formulations have grown in popularity as an alternative to coatings containing biocides. These coatings are designed to create smooth, non-stick surfaces to which fouling organisms cannot adhere. Note, however, that these formulations can be easily damaged, are hard to repair and may not always prevent microfouling from attaching to surfaces.

As with any other coating job, consult with coating manufacturers and examine material data sheets prior to application.

Proper application of antifouling coatings

The risk of fouling is inherent due to common design elements on ship hulls, including welding seams, valve openings, bulging plates or any other surface characteristic that fosters growth of fouling organisms.

But the most common cause of fouling on a ship hull can be traced to previous antifouling coatings that were poorly applied. The single greatest driver of success in antifouling is applying the coatings correctly, and proper application is all about timing.

After a primer is applied to a surface, specs may call for one, two, or even three intermediate coatings before antifouling is applied. The best time to apply antifouling is when the final intermediate coat (also known as the tie coat) is tacky to the touch—wet enough to leave a fingerprint in but dry enough not to stain fingers.

This timing is crucial as antifouling coatings work best when they dry with the coat that precedes it.

The best way to assure antifouling coatings are applied properly is to vet coating contractors thoroughly and choose one with a demonstrated reputation of getting it right.

AMPP strives for safety and success in the maritime industry by offering the nationally recognized QP 1 accreditation program. This program provides facility owners and specification writers a means to determine whether the painting contractor has the capability to perform surface preparations and coating application in the field on complex industrial and marine structures.