Boat Repair: Common Techniques Discussed
1. Osmotic Blister Discussed
2. Gel-coating Over Epoxy
3. Blistering Damage
4. Dealing with Rot
1. Osmotic Blistering
Osmosis- Movement of a solvent through a semi-permeable membrane into a solution of higher solute concentration that tends to equalize the concentrations of solute on the two sides of the membrane.
On immersion in water all coatings absorb water, 0.1 to 3%. Water seems to gather around interfaces in the coating, intercoats, pigment binder, polymer metal, but especially at the polymer metal interface. This Absorption causes swelling of the film which then expands locally producing a loss in adhesion. This effect depends on water absorption, film cohesion and adhesion. In this instance the blisters would not be filled with water. The presence of retained solvent which was hydrophobic at the film substrate interface is likely to enhance this effect.
Blistering and Dis-bonding
Blisters are local defects in a a surface usually at the outer surface interface formed by an accumulation of water or aqueous solution together with a loss in adhesion and stretching of the outer layer.
Mechanisms of Blistering
There are a variety of way in which blistering might be thought to initiate. Here are five possibilities which are not necessarily mutually exclusive.
Subsequent Corrosion
In the absence of determining barrier effects, there is always sufficient oxygen and water present within a blister for corrosion to proceed. Furthermore, the water soon becomes conductive either because of the presence of ions on the surface or by the leaching of conductive material from the substrate itself. So we have in effect within the same blister, reactive elements, electrolyte, oxygen and water. Corrosion is therefore spontaneous and proceeds. Soon physical separation of anodic and cathodic site takes place, with anodic sites predominating at the center of the blister and cathodic site predominating at the edge of the blister. The cathodic process generates alkali which penetrates under the blister and allows it to expand and detach the outer surface or film and the blister grows. Frequently the single blister is the nucleating site for subsequent blisters which nucleate at the edge of the dis-bonded front.
2. Gel-Coat Repair Techniques
Many of the questions concerning the repair of fiberglass boats seem to revolve around the techniques used when applying polyester gel-coat to cover a repair made with WEST SYSTEM® epoxy. There are several steps to a successful repair. It is important that these steps are done in the proper order to assure a well matched repair color.
One of the steps that is frequently left out is to apply a sealer coat of epoxy to the repair area. This is necessary to fill any porosity in the patch. Apply two or three very thin coats of epoxy, extending each coat slightly beyond the previous one. Warm the area with a heat lamp to speed the cure and to help the epoxy flow out nicely.
When the sealer coat has cured, wash thoroughly with water and a 3M Scotch-brite pad to remove any amine blush. Any blush left on the surface may inhibit the cure of the gel-coat This very important step is often forgotten, and the result is usually an unsatisfactory repair.
Sand the epoxy coated repair area with progressively finer grits of sandpaper. Finish with 220 wet or dry.
De-wax an area twice as large as the diameter of the repair. Apply tape around the perimeter of the de-waxed area. Use masking paper to protect the boat from over-spray If there is a molded body line or corner near the repair, you may want to extend the color patch to that point. The same is true for a painted or vinyl stripe.
Sand the additional area out to the tape line with 320-grit paper. This will be the total area to be gel-coated
Determine the gel-coat batch size for the size of the repair—approximately 80 square feet per gallon of gel-coat (20 sq. ft. per qt.).
Tint the batch of gel-coat to match the color of the boat. If the boat is fairly new and the manufacturer is still in business, you may be able to get gel-coat that will be a very close color match. If this is not an option, you will need to get the gel-coat from a FRP product supplier. Many marine distributors handle the gel-coats from various resin manufacturers. You will also need pigments to tint the base color to obtain a good match. These pigments are generally available from the gel-coat suppliers. Frequently, local repair yards that do fiberglass repairs will sell these products to the project user.
There are several gel-coat additives that, when used properly, make matching a repair easier. These are clear, low-viscosity resins which are not air inhibited. These products are mixed with the pigmented gel-coat in place of a wax solution to provide a tack-free cured surface. They also provide the added benefit of thinning the gel-coat without changing the color of the cured patch the way acetone or styrene can. A few such products are Duratec Hi Gloss Clear product number 904-001; Fast Patch by American Colors; and Cook Paint and Varnish Company's number 970-X-900 Speed Patch Additive.
Matching the repair color to the color of the boat is probably the hardest part of the entire job. You will find that most gel-coat colors change as they cure. As you tint the gel-coat to match, apply a small amount of un-catalyzed material to the sanded area surrounding the repair. Use your gloved finger to spread this into a sample the size of a quarter. Wait a couple of minutes for the solvents to flash off. Any color variation will be evident. If the color match is not acceptable, change the color by adding small amounts of tinting pigments. As you adjust the gel-coat color, try to think in terms of the basic colors, i.e.; the color needs to be blacker or redder or bluer or greener... This will help you identify which of the pigments to use. Use very small amounts of the pigments. You may even need to dilute the pigment with white gel-coat to weaken the effects. Apply an un-catalyzed smear with each change of color no matter how small. Leave each of these smears on the surface until you have the color as close to the boat color as you can get it. When you are satisfied with the color match, wipe away all of the test smears with acetone or lacquer thinner.
Divide the batch into a 2/3 portion and a 1/3 portion. Our technique for using the patching additives is slightly different than the recommended procedure. Mix the patching additive with equal parts of the 2/3 portion of the matched gel-coat Catalyze following the recommendations of the gel-coat supplier. Over or under catalization may prevent the product from reaching a proper cure. Apply this mixed gel-coat to the surface of the repair with a spray gun. Apply several light coats, feathering each one farther from the repair area. Allow the solvent to flash off between coats. You may need to apply five or six coats to hide the shadow of the repair. Most gel-coats colors will dry lighter in color if they are too thin. Most manufacturers recommend a total film thickness of 15 to 20 mils. A common mistake is to apply two or three heavy coats, causing solvent entrapment and improper curing. This may also have an effect on the color of the cured repair.
Make another mixture with the 1/3 portion of gel-coat and patching additive. This time use about three parts of patching additive with one part gel-coat Spray the first coat of this mixture over the previously sprayed area. Then apply two or three more coats of this mixture over the entire area. Extend each coat farther than the previous one, with the last coat out to the tape line. The semitransparent film of lightly tinted clear patching additive will allow the original gel-coat color to show through around the perimeter of the patch. This will help blend any subtle color change into the original gel-coat
Once the gel-coat has reached a full cure, pull the tape and sand the tape line smooth. Start with 320-grit wet or dry paper and work up to 600-grit. Sand the surface of the repair if necessary.
Buff the surface with a white, medium-cut rubbing compound. Take your time and do not overheat the surface while buffing. The excess heat can cause a stain which will be very difficult to remove. Finish with a fine compound and wax the area.
There are times when the repair will not match simply because the original gel-coat color has faded. You may need to polish the entire side of the boat to restore the original color.
Patience and experience are the two most important ingredients for a well matched color repair. The mixing of the color is guaranteed to teach patience. And since you may need to try more than once for a proper match, the experience comes quickly. So get to it.
COPYRIGHT ©1991 Gougeon Brothers, Inc., PO Box 908, Bay City, MI 48707, USA. WEST SYSTEM is a registered trademark of Gougeon Brothers, Inc.
3. Blistering Damage
The first sign of an osmosis, or more adequate hydrolyze, process is that small blisters become visible on the hull. Even if they don't appear to be anything but a cosmetic issue, the underlying damage is quite severe.
When the "osmosis" blisters become visible, a complete layer of the laminate has been severely affected.
Alkali residues from the hydrolyze will be spread inwards in the fiber glass capillaries and cause an almost total loss of bond between the fibers and the polyester!
4. Chemotherapy for Rot
by Dave Carnell
Once rot gets a toehold in wood it is difficult to cure completely—it is like a cancer. Digging out the rotted wood will still leave spores and water in the sound wood. After you fill in the cavity with something like epoxy, the rot continues to flourish underneath. Products promoted to make rotted wood sound and stop rot penetrate only until they meet water, with which they do not mix. Under the solid repair rotting goes on. With one exception (more later), the commercial products sold to treat dry wood to prevent rot are completely ineffective against established rot in wet wood because they are dissolved in petroleum solvents and oil and water do not mix.
There are two commonly available inexpensive materials that will kill rot in wood and prevent its recurrence. First, there are borates (borax-boric acid mixtures) which have an established record in preventing rot in new wood and in killing rot organisms and wood-destroying insects in infested wood. Second, there is ethylene glycol, most readily available as auto antifreeze-coolant. Glycol is toxic to the whole spectrum of organisms from staphylococcus bacteria to mammals. All of the published material on its effectiveness against wood-destroying fungi and insects that I am aware of is the result of my investigations over the past 15 years.
Both borate solutions and glycol penetrate dry and wet wood well because they are water-soluble; in fact, penetration by glycol is especially helped by its extreme hydroscopicity—its strong attraction for water. For both, the fact that they are water-soluble means they are not permanent solutions to rot in wood that is continually exposed to water-below the waterline and in ground-where they will eventually be extracted-dissolved out.
I first was interested in glycol as a wood-stabilizing agent, where it is in many ways superior to polyethylene glycol (PEG), and it was during this work that I realized the useful effect of glycol on organisms, though I was pretty dense in interpreting the first experiment.
The ladies immerse the stems of greenery such as magnolia branches in glycerin to keep them green. Glycol is very similar to glycerin in all its physical properties and much cheaper, so I stuck a magnolia branch in antifreeze. The next day it was brown. After the third attempt I tumbled to the fact that the glycol was killing the greenery.
This was the reason that glycol never replaced glycerin in applications such as a humectant for tobacco and an ingredient of cosmetics and pharmaceutical ointments, though it had all the desirable physical properties.
I had two 2" thick slabs of a 14" diameter hickory tree that had just been cut. I treated one with antifreeze and left one untreated. I was looking at wood stabilization, not rot prevention. After about six months stored inside my shop the untreated control was not only cracked apart, but it was sporting a great fungal growth, while the treated slab was clean.
The local history museum wanted to exhibit two "turpentine trees", longleaf pines that had many years ago been gashed to harvest the sap that made everything from turpentine to pine tar. The trees delivered to us after cutting were infested with various beetles and had some fungal growth. I treated them with antifreeze outside under a plastic tarpaulin every few days for three weeks. They were then free of insects and fungus and have remained so after being moved inside and installed in an exhibit over four years ago.
I took three pieces from a rotting dock float that were covered with a heavy growth of fungus, lichens, etc. I treated one with antifreeze painted on with a brush, the second with a water solution containing 23% borates (as B2O3), and left the third untreated as a control. They were left exposed outdoors and were rained on the first night. By the next morning the growth on the antifreeze-treated piece was definitely browning and the borate-treated piece showed slight browning. After two months exposure to the weather the growth was dead on the antifreeze- and borate-treated pieces and flourishing on the control.
I have a simple flat-bottomed skiff built of plywood and white pine, which has little resistance to rot. After ten years some rot developed in one of the frames. It may have begun in the exposed end grain. It consumed the side frame, part of the bottom frame, and part of a seat brace fastened to the side frame. The plywood gusset joining the side frame to the bottom frame was not attacked. I excised the rotted wood, saturated all with ethylene glycol antifreeze to kill all the rot organisms, and there has been no further deterioration in four more years afloat with wet bilges. I have not replaced any pieces, as I am building another boat that can replace it; that is more fun, anyway.
I have a 60+ year old case of the fungus infection known as "athlete's foot". Many years ago it infected the toenails extensively. The whole thing was pretty grotesque. My dermatologist and druggist both assured me there is no known cure. About six years ago I started using antifreeze applied under the nails with a medicine dropper about every five days. The professionals are technically right. I have not completely cured it, but the nails have grown out pink and thinned almost to the ends and I never have any trouble with blistering, peeling, or itching between the toes as I had had for six decades. No drug company is going to have any interest in this because the information has been in the public domain for so long that there is no opportunity for any proprietary advantage. The various wood-rotting organisms cannot be anywhere near as tough.
Glycol by itself has one big advantage over solutions of borates in either water or glycol. Glycol penetrates rapidly through all paint, varnish, and oil finishes (except epoxy and polyurethanes) without lifting or damaging those finishes in any way. You can treat all of the wood of your boat without removing any finish. The dyes in glycol antifreeze are so weak that they do not discolor even white woods. Once bare wood has been treated with glycol or the borate solutions and become dry to the touch it can be finished or glued. If a borate solution leaves white residues on the surface, it will have to be washed off with water and the surface allowed to dry.
This is my preferred process to treat rot. Once you find soft wood or other evidence of rot, soak it with antifreeze even if you cannot do anything else at the moment. Paint it on or spray it on with a coarse spray. Avoid fine mist-like spraying because it increases the likelihood that you will breathe in unhealthy amounts of glycol. Put it on surfaces well away from the really damaged wood, too. Use glycol lavishly on the suspect wood, which will readily absorb 10-20% of its weight of antifreeze.
Next dig out wood that is rotted enough to be weak. Add more glycol to wet the exposed wood thoroughly. Then add the 25% borate solution of the recipe below so long as it will soak in in no more than 2-3 hours. Then fill in the void with epoxy putty and/or a piece of sound treated wood as required. The reasons I use borates at all are: 1) it is a belt-and-suspenders approach to a virulent attack, and 2) over a long period glycol will evaporate from the wood; especially, in areas exposed directly to the sun and the high temperatures that result.
If there is any question about water extracting the glycol or the borates, you can retreat periodically with glycol on any surface, painted or bare, and with borate solutions on bare wood.
Glycol's toxicity to humans is low enough that it has to be deliberately ingested (about a half cup for a 150 lb. human); many millions of gallons are used annually with few precautions and without incident. It should not be left where children or pets can get at it, as smaller doses would harm them, and they may be attracted by its reported sweet taste that I have confirmed by accident. The lethal dose of borates is smaller than of glycol, but the bitter taste makes accidental consumption less likely.
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