Gelcoat Blisters
When fiberglass boat building started, very little thought was given to the possibility that the fiberglass or the tough gelcoat surface might eventually break down. Gelcoat was developed to protect the fiberglass structure and was thought to be an impermeable layer that would keep the underlying resin and fibers dry for the life of the vessel.
As time has borne out, blisters (osmotic blisters) appear on some boats; sometimes quite quickly, and in other vessels only after many years of use. Water in warmer climates seems to encourage them (with the proportional longer boating season) and the builder’s specific choice of resin may also help deter them. The fact is, to date, we can’t accurately predict when a blistering problem might occur.
To understand more about blisters here is a quick review how the typical fiberglass boat is constructed. First, mold release wax is sprayed onto the inside of the mold, then the layers of gel coat, a specially formulated pigmented polyester resin that gives the boat its color and smooth exterior finish, are sprayed on. Laminated above the gel coat are layers of fiberglass, usually in the form of chopped-strand mat and woven roving, saturated with polyester resin. It is inside the laminated fiberglass matrix that osmotic blisters can begin when water seeps through the gel coat, seeking out pockets of incompletely cured resin within the fiberglass layers.
Tiny amounts of water get inside the fiberglass and begin to dissolve the chemicals found there. This solution then attracts additional water through the gel coat barrier by the process of osmosis. As more water enters, the pressure increases and eventually, a blister is formed. Under the right conditions this blister might expand and begin to delaminate the local area around the blister. As more blisters occur, the situation can become a problem that might threaten the structural integrity of the hull. Cored hulls present a complex problem in that both the core material and the fiberglass/ gelcoat may be damaged in extreme cases.
As builders determined that blisters resulted from water penetrating the gel coat surface, the barrier coat (usually an epoxy, or similar material that seals out the moisture better than polyester gel coat) was born. Water barrier techniques and products have grown into maturity over the past few decades.
Applying a barrier coat today is almost as easy as rolling on bottom paint, and thousands of boat owners have completed this project, protecting their boats from osmotic blisters. There are three types of barrier coat protection: 1) preemptive barrier coating on new boats; 2) blister repair alone; 3) blister repair with a barrier coat.
New Boat Preemptive Barrier Coating
New boats are sometimes delivered with a barrier coat applied at the factory, and may even include a 5 or 10 year blister warranty. It is important to note that you may have to treat the barrier coat in a specific way to keep the warranty intact, so consult your warranty before you do a thing to the bottom of your new boat. 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 in this instance– this type of resin is considerably more costly) so that you may not have to apply a barrier coat to the underwater portion of the hull.
If the vessel has no special barrier protection and your boat is in the water year round, especially in a warm-water area, or if other boats from the same manufacturer (or of your model) have a history of blister problems, barrier coating as a preventative measure makes a lot of sense. 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 properly applied barrier coat may also increase the resale value of your boat.
If you want to take this preemptive action, then wipe-down the bare hull with a solvent dewaxer, sand thoroughly, but lightly, and apply the recommended thickness of barrier coat (usually 2 or 3 coats), followed by antifouling paint appropriate to the area’s waters.
Hamilton Marine offers barrier coat products from a number of paint manufacturers. A review of their manuals will reinforce how seriously they take this problem. They give drying times along with precise recommendations as to the number of coats that will give the optimal film thickness, after it dries. Follow these instructions closely and you will have satisfactory results.
Blister Repair
Many times when a boat is hauled you’ll see isolated blisters on the hull, or possibly on the rudder, but in general the gel coat 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 the much more costly barrier coat application for a few more years—with luck, forever!
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. Mark the blisters, so you don’t miss any later. Open the blisters by scraping or grinding away the damaged material until there is no sign of delamination. Flush with water and allow the damaged areas to dry, which could require from several days to a few weeks. Circulating warm air will speed the drying process.
When the cleaned out areas are dry, the first step is to seal the laminate with low viscosity 100% solids epoxy resin. This coat penetrates the surface and prepares it for the epoxy filler, which is spread in 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 regular density epoxy (do not use 5-minute epoxy, as this is not waterproof) will seal the filler. Sand after curing and you’re then ready for bottom paint.
Blister Repair with a Barrier Coat
Older boats 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, either through natural drying (very slow), by artificially warming the hull (expensive), or by using a vacuum to draw out the moisture (technically complex and expensive). Any moisture that is resident in the hull laminate will be trapped by the barrier coat, if it is applied prematurely. Avoid doing this—or all your efforts will be in vain. Sever problems might require stripping the gel coat 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. Six months or more is not unheard of.
The coating procedure is a combination of the first two procedures above: coat the areas of the hull where the gelcoat was removed with unthickened epoxy, fill the holes with epoxy filler and fair, and then apply 4–6 coats of the barrier coat material to the outside of the hull. 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 ideal conditions, and it produces a better bond between coats of barrier coat epoxy.
More Help
Hamilton Marine recommends and sells a number of good books on hull repair: Don Casey’s Sailboat Hull and Deck Repair and This Old Boat are both good resources. We also recommend two introductory publications from Gougeon Brothers’ series of WEST SYSTEMS books: Fiberglass Boat Repair and Maintenance and Gelcoat Blisters: Diagnosis, Repair, and Prevention. They are inexpensive and full of information. For free information on the topic you also may want to check online at www.yachtpaint.com, or www.pettitpaint.com. You’ll also find up-to-date information based on current experience can be found in many boating magazines and on other web sites. Analyzing and following procedures outlined in these publications and web sites should help you find a solution to your boat’s particular blister problem.
