Case Study –Applying Antifoulant Undercoating to an Aluminum Hull Vessels

The NOAA research vessel the Peter Gladding is a 56 ft high-speed aluminum catamaran that patrols the Dry Tortugas Sanctuary in the Florida Keys.  The vessel came to the 5th Street Marina for underbody recoating and other miscellaneous repairs.  Upon hauling and cleaning, it was observed that the existing antifoulant coating had failed on over 50% of the hull after only one year of use.  The following describes the probable cause of the coating failure and the preventative measures that were taken to make sure that recoating would adhere successfully.

Existing Coating Condition

After pressure cleaning the marine growth off the hull, the underbody coating was inspected.  What was left of the remaining system was an intact layer of epoxy primer and two partially adhering topcoats. The topcoats were brittle and easily flaked off. It was clear that the antifoulant paint had not adhered properly to the underlying primer layer. On a typical recoat, the antifoulant would be reapplied over the existing layers, however if the existing antifoulant is not adhering to the primer, additional layers will also fail. To remedy this condition, the underbody primer must be reapplied.

Recoating Process

To properly apply the primer layer, the existing antifoulant paint must be completely removed.  Since the existing primer was intact and appeared to be properly bonded to the metal, there is no need to remove it. The less evasive low-pressure sweep blast was used to prepare the surface by removing the topcoats and exposing the primer layer for recoating.

The  owner requested the use of Sherwin Williams’s products.  Sherwin Williams Seaguard 5000HS Epoxy was used for the primer.   SeaGuard Heavy Metal Free Ablative Antifoulant was used for the topcoats.  The SeaGuard line is approved by the Coast Guard for high-speed applications on aluminum hull vessels. A red color was used for the initial topcoat and then two coats of black. Each coat was to be applied to a dry mil thickness of 5 mils.

Surface Temperature Issues

While detailing the hull for sand sweeping, it was noticed that the aluminum would get extremely hot in the sun.  Temperatures were recorded over 130 degrees.  In addition, one of the other miscellaneous items performed on the vessel was the insulation of the interior berths.  The Captain had noted that during the day, the vessel air conditioning could not keep temperatures in the cabins below 80 degrees.  The aluminum hull and deck was conducting heat at a much faster rate than a typical fiberglass hull vessel.  The high surface temperature would be an issue during recoating.

When applying the underbody coating, the epoxy layer must be allowed to dry but not cure. The primer must be soft to touch and still slightly tacky. The first coat of antifoulant must then be immediately applied. This allows the antifoulant and primer to cure together creating a chemical bond between layers.   With an aluminum hull vessel like the RV Peter Gladding, the rapid high levels of heat transfer from direct sunlight would cause the primer to dry-out faster and rapidly cure.  Unless controlled, the primer layer would cure before the antifoulant could be applied. This is likely why the original coating failed prematurely.

Ideally, the vessel would be painted indoors or underneath an enclosure that would shield the vessel from direct sunlight so that the surface temperature could be controlled. Since the vessel was already blocked and the on-site hangers were in use, a plan was made to use several personnel during the recoat process to ensure that the whole vessel would be primed and the initial layer of antifoulant applied on the same day.  The weather, humidity, and cloud cover must also be considered when coating outdoors. The first layer of antifoulant paint was allowed to dry overnight. Upon review the next day, the initial layer showed good adhesion with the epoxy primer layer. A second coat of black colored antifoulant was then applied. The following day the final layer of black coating was applied.

The owner was contacted six months after this work was completed and informed us that there have been no more problems with adhesion and that the antifoulant was performing successfully.


Aluminum hull vessels will conduct heat through the hull much faster than a fiberglass counterpart.  Care must be taken when performing underbody paint work to shield the  surface from direct sunlight or take other precautions to prevent the premature curing of the bonding layer before the application of the antifoulant coating or otherwise the coating system will eventually delaminate.

Written by Orin Black, General Manager of the 5th Street Marina