Epoxy Barrier Coats

Grinding out blisters on the bottom of a boat

Grinding out a blister and beveling the edge of the repair area to blend into the surface.

By Tom Burden, Last updated: 8/22/2019

What are osmotic blisters?

In the 1950s, when fiberglass boatbuilding started, no one thought about the possibility of gelcoat blisters. Gelcoat was believed to be an impermeable layer that would keep the underlying resin and fibers dry forever. But, as we now know, blisters mysteriously appear on some boats and, just as mysteriously, not on others—sometimes quickly and sometimes after years of use. Warm water seems to encourage them and the exact choice of resin may help deter them, but the fact is—even today—we cannot predict when a blistering problem might occur.

To understand how to avoid and to deal with blisters, let’s quickly review how the typical fiberglass boat is constructed. First mold release wax is applied to the inside of the (usually) female mold, then the layers of gelcoat, a pigmented resin that gives the boat its color and smooth exterior finish, are sprayed on. Laminated over the gelcoat are layers of fiberglass, usually in the form of chopped-strand mat and woven roving, saturated with polyester resin. It is inside this fiberglass matrix that the blister mischief begins when water seeps through the gelcoat.

Tiny amounts of water get inside the fiberglass and begin to dissolve the chemicals found there—generally uncured resin or contaminants from the boatbuilding process. This solution then attracts additional water through the gelcoat barrier by osmosis. As more water enters, the pressure increases and, finally, a blister is formed which can actually delaminate the local area around the blister. As more blisters occur, you can rapidly move from an unattractive nuisance to a problem that threatens the structural integrity of the hull.

Sealing the Water Out with Epoxy

Once manufacturers figured out that blisters resulted from water penetrating the gelcoat membrane, it didn’t take long to figure out what to do about it. The barrier coat—usually epoxy or similar material that seals out the moisture better than polyester gelcoat—was born. Over the last 30 years or so, techniques and products have grown to maturity.

Applying a barrier coat is often about as easy as rolling on bottom paint, and boat owners by the thousands have completed this project, protecting their boats from osmotic blisters. There are three categories of projects that you may encounter in this area: preemptive barrier coating on new boats, blister repair alone, and blister repair with a barrier coat.

Do you have a new boat?

New boats are frequently delivered with a barrier coat applied at the factory, and may include a five- or ten-year blister warranty. It is very important to note that you may have to treat the barrier coat in a specific way to keep the warranty intact (there may be a prohibition against sanding it, for example). If your boat does not have a barrier coat, you may want to investigate whether the builder has used specific resins which are resistant to osmotic blistering (commonly vinylester resins are used) so that you don’t feel obligated to coat your new bottom with goo.

Preventive Barrier Coating

If the builder has not added this protection and you keep your boat in the water year round, especially if it is in a warm-water area, or if other boats from the same manufacturer have a history of blister problems, barrier coating as a preventive measure makes sense. One thing is certain, it is far easier to apply a barrier coat to an intact hull than to clean out and fix blisters first, and then roll on the epoxy. A barrier coat will also increase the resale value of your boat. We offer several barrier coat products. WEST System Epoxy and the InterProtect 2000E system from Interlux are very popular. We also have Pettit Protect and the low-VOC InterProtect 2000 system. All of these are effective.

If you want to take this preemptive action, wipe-down the hull with solvent dewaxer, sand thoroughly, and apply the recommended thickness of barrier coat, followed by antifouling paint appropriate to your area’s waters. Barrier coat manufacturers provide very detailed application instructions. They give drying times, literally by the hour, for the range of temperatures over which the coating may be applied, along with precise recommendations as to the number of coats that will give the necessary film thickness after it dries. And film thickness is what counts: a coat may provide 3 mils (0.003") of thickness when dry, and 12–20 mils of coating may be required.

Are you stripping the antifouling paint this year?

If your Spring Commissioning involves a haulout with removal of the antifouling paint, it represents a golden opportunity for a quick status check on the state of your gelcoat and the laminate underneath it. If your boat has been out of the water for the winter, that’s another positive, because the hull may have dried out while on the hard. A dry hull (measured with a non-invasive moisture meter) is one of the keys to a successful barrier coat. You want to seal moisture out, not lock it in with impermeable epoxy.

scuffing blisters with a sanding disk to highlight them before they shrink

This porous modified epoxy antifouling paint has been scuff-sanded with a disc grinder to highlight the blisters before they dry out and shrink.

Should you just repair the blisters?

Many times when a boat is hauled you’ll see isolated blisters on the hull, or possibly on the rudder, but in general the gelcoat looks smooth and there is little evidence of major damage. In these cases, it may only be necessary to fix the blisters and put off applying the much more costly barrier coat until a future date.

As soon as the boat is hauled, scuff-sand lightly over the blisters before they ooze all of the liquid out, deflate, and become hard to identify. Open the blisters by scraping or grinding away the damaged material until there is no sign of delamination. Then flush with water and allow the damaged areas to dry, which could require from several days to a few weeks.

When the blistered pockets are dry, the first step in restoration is to seal the laminate with low viscosity 100% solids epoxy resin. Almost like varnish, this coat penetrates the surface and prepares it for the epoxy filler, which is troweled into place to fill the void. Deep pockets, or large areas, may require a few coats of epoxy filler compound, although the material will cure even if it is applied in a thick layer. When the void is filled, sand the surface to match the surrounding hull contours. A final coat of resin/hardener mixture alone will seal the filler, and you’re then ready for bottom paint.

Blister Repair with Application of a Barrier Coat

Older boats, with or without obvious blisters, must be very dry before you attempt to repair them. By dry, we mean that the hull laminate must have the water evaporated from it, through natural drying (a desert location helps), by artificially heating the hull, or by using a vacuum to draw out the moisture. Any moisture that is resident in the hull laminate will be trapped by the barrier coat if it is applied prematurely, and this must be avoided.

One school of thought prescribes stripping the gelcoat entirely, testing the laminate with a moisture meter and letting the boat sit on the hard for as many months as it takes for the meter to read the same both above and below the waterline. Yes, we said months. This is a slow process (much like watching golf or baseball on television, or having to listen to sailors describe the day’s race at a yacht club bar).

The coating procedure is a combination of the first two: coat the hull with unthickened epoxy, fill the holes with epoxy filler and fair, then apply 4–6 coats of the barrier coat material. Putting on all of these coats can be greatly speeded up by coating “wet on wet”. This means that you wait until the layer you are overcoating has reached a partial cure, but is still a little “tacky”. If you roll on another coat at this point, the two layers will chemically bond, and you won’t have to sand between coats, or wait for each coat to cure completely. This may allow three or more coats per day under the right conditions.