More on hull barrier coats

I read with interest the recent letter from Marci and J. Kolb who had problems with blistering on their Hinckley 38 (Barrier-coat problems require extended fix Issue 132 Sept./Oct. 2003). Having not seen their boat or talked to them, I cannot comment on the problems they had except to say that osmotic blistering is a complicated chemical process that occurs when water gets into the laminate. The water enters the laminate in the form of water vapor — which will penetrate all polymeric materials.

Once water permeates the gelcoat and reaches the laminate layers, it forms concentration cells in the voids and draws in additional moisture. It then begins to break down the resin in the laminate by hydrolyzing the polyester back to its constituents despite the cross-linking of the resin. This creates more solutes and draws more water into the cell. The liquid solution behind the gelcoat seeks to reach equilibrium with the water on the outer skin of the gelcoat. During the attempted equilibrium process, pressure increases and distends the gelcoat. On some hulls the gelcoats are very thick and do not blister; instead they crack, but whether blistered or cracked, the hulls still absorb water. In simple terms water passes into the laminate and begins to break down the polyester into the various esters from which it is made. These materials are hygroscopic and draw more water to them, and the process continues. Pressure builds up, and the gelcoat blisters or cracks. Not only can water vapor pass through the gelcoat from the exterior, it can also absorb water from the bilge.

The three most important things that affect the longevity of a blister repair or prevention procedure are, 1) getting the surface prepared correctly, 2) getting the laminate dry and 3) getting the right amount of epoxy on the boat. For new or nonblistered boats, we recommend they be cleaned and then sanded with 80-grit sandpaper, and that 10 to 12 mils of InterProtect 2000E or 3000 be applied.

Boats that have blistered will have to be dried prior to application of any product. Not only must the water be removed but also the water-soluble materials that remain in the laminate. These will usually weep to the surface but do not evaporate as does water and must be removed physically by washing with water since they are water-soluble. On severely blistered boats it is usually necessary to remove the gelcoat and the affected laminate layers to get the boat dry. This is accomplished by peeling, grinding or sandblasting. The best way to check on hull dryness is to use a moisture meter. We have always recommended that, prior to overcoating, the readings be less than 5 on the A scale of a Sovereign Moisture Meter.

Where a boat is used is also a factor in determining the length of time a blister repair will last. Fresh water, being less ionic than salt water, more easily passes through a semipermeable layer such as gelcoat. Warm water also moves more easily into the laminate. So the worst place for blistering is warm fresh water. The longest-lasting repairs are in places like Maine, where the water is cold and salty, and the season is short. The Kolbs’ boat was in the water four months of the year when they were in Maine, and even a hull that has not been dried properly most likely will not reblister under those conditions. If you put the same boat in warm water and leave it in for two years, it will most likely reblister.

The Kolbs also mention some problems they had with InterProtect 1000. InterProtect 1000 is a 100 percent solids clear epoxy and like most 100 percent solids clear epoxies, it is an excellent adhesive. There may have been some contamination left on the surface, but without looking at the boat it would be difficult to tell. InterProtect 1000 is used to fill empty laminate and fibers prior to filling and fairing, and because it is 100 percent solids, solvents cannot be trapped behind it as it cures. InterProtect 1000 is made to be very thick and gooey, so that when you are working underneath a boat, it is less likely to drip, but this makes it more difficult to apply. “Orange peel” is caused by poor flow, and with a thick product like InterProtect 1000, you may see more of it than with a thinner product. Our new Epiglass Epoxy flows better and more easily wets out the surface, and results in less orange peel. Clear epoxies, such as InterProtect 1000, are amine-cured, which can lead to a chemical blush forming on the surface. If the blush is not removed, it gums up the sandpaper, making it difficult to sand the surface. The amount of blush varies depending on temperature and humidity during the cure time. Amine blush is removed by scrubbing with soap and water.

InterProtect 2000E is a very hard and durable epoxy coating that uses the same chemistry used in icebreaker coatings, so I don’t understand Mr. Kolb’s comment about InterProtect lasting only seven years. InterProtect has been sold since 1986, and while nothing lasts forever, there are boats that have been painted since then that have not blistered or reblistered. The confusion may arise from the fact that there used to be an insurance company that had a program where customers that had their boat repaired at an Interlux-approved yard could get an insurance policy against the hull reblistering. To maintain the insurance, the customer would have to have the antifouling removed from the hull and more InterProtect 2000E applied every five years, but that condition was imposed by the insurance company, not Interlux.

It concerns us when a customer has a problem with one of our products, and it would have been in the best interest of the Kolbs and Interlux to have one of our field representatives look at their boat and make some recommendations. Should any of your readers have a problem or a question, we can be reached at 800-468-7589 or at

Jim Seidel is assistant marketing manager of Interlux Yacht Finishes at International Paint Inc., in Union, N.J.

By Ocean Navigator