The Smith Method
of Wood Boat Bottom Restoration

April 2021
Steve Smith

5100 Channel Ave Richmond CA 94804-4646 U. S. A. Vox 1-510-237-6842 fax 1-510-232-9921

Starting back about 1972 I was selling a variety of adhesives, sealants and coatings in the San Francisco Bay area from my one-man business there, and was doing no advertising at all, being only a chemist and engineer and knowing nothing of marketing. I was an aerospace electronics engineer and this paint-and-glue business was really a sideline, a sort of hobby for me. People with various kinds of boats heard about me and came to my shop with their problems. I often went to the local boat yards and looked at their boats with various afflictions, and occasionally concocted something special. Some of these eventually became standard Smith & Co. products.

In the middle seventies, blistering of Glass-Reinforced Polyester [GRP] boat hulls was common, and I was the one who identified the fundamental cause [water decomposes esters, and polyester is that kind of resin] and invented the moisture-diffusion-barrier-coat that everyone now uses to repair osmosis-blister damage and prevent it from happening. The details of how and why this works and how to treat your GRP hull is too long to go into here, but anyone who wants to know more can e-mail me or call my factory at 1-800-234-0330; the procedure is now posted on the website.

Anyway, I had identified products and technology that kept water from getting into a GRP boat hull, and between reducing the rate of water uptake from the side of the GRP hull in the water, and facilitating evaporation of water off the inner hull surface [improved ventilation, dealing with integral tanks, etc.], I had created a situation where the hull stayed dry, even when floating in the water. It was not uncommon for a GRP hull redone with my procedures to float several inches higher a year later, as the water-saturated resin laminate slowly dried out from the now-ventilated inside even while floating. This same result would eventually be attained with wood hulls.

In the early eighties some friends of mine from Sweden were visiting me in America because I was one of the founders of an international franchise that used certain magical primers I invented.......I can't say what it is but many people have one in their home, mainly in the U. S. These folks were the European Master Franchise holders. Looking for more of my products to market, they got interested in the GRP blister repair technology. They licensed it, set up franchises in Europe to apply the coatings and actually had a warranty on the result that was backed by Lloyds of London.

You're probably wondering how wood boats got into the picture, and it came about this way: In spite of my having had MANY wood-boat-owners for customers over the previous ten years, I only had Clear Penetrating Epoxy Sealer for restoration of aged wood and 2-component Polysulfide rubber for sealing the seams. People commonly applied antifouling paint on top of that. Some applied my High-Build Epoxy Paint moisture-diffusion Barrier Coating on top of the wood and before the antifouling paint, but I never really appreciated the significance of that entire coating system at the time.

My Swedish friends, having gotten the GRP repair business established, began to look further for other things that could be done with these materials they were importing into Europe. They told me of the typical Swedish wood boat, which is hauled out for about six months of the year, the wood dries out, the planks shrink, and the seams open. Then, when launched in the spring, it takes about three days for the wood to swell and sort-of-seal the seams, during which time fantastic amounts of water leak into the boat, requiring constant attention lest the vessel sink. They asked me what I had that could be done and whether this represented a market for my products there. Having now clearly understood the function of the moisture-diffusion-barrier-coating from the work with GRP hulls, I thought to apply the same technology to solve the problems of leaking seams in wood boats, and began to work out how to do it.

The fundamental difference between wood and GRP hulls was that wood hulls had seams, and planks that changed in width with variations in weather, in-and-out-of-the-water, and temperature. I realized that putting anything in the seams was going to be a problem, as the planks would try to compress the solid material or even rubber, and stretch the screws, probably breaking or tearing-loose some, and in extreme cases of packing solid rubber in the open seams of a dried-out -hull, when the wood swelled it could literally jack-out the chines, which is kinda destructive. This would not be good.

Wood was stronger when not waterlogged, and so there seemed to be two stable states for which to strive: Wood in equilibrium with the moisture in the air and wood in equilibrium with the moisture in the water. The latter seemed less desirable, and if the seams could be sealed while allowing small movements of the planks, then there was the potential for a wood boat with a planked hull to actually float in the water with a dry bilge. The unheard-of would eventually become commonplace.

My thought at this point was to pilot wood-boat-restoration in the U. S., where I could observe more closely how this worked before releasing the technology for use half-a-planet away. The problem to solve was how to keep water from leaking through a seam between two planks in spite of variation in the seam width from e.g., zero-to-an-eighth-of-an-inch]. The problem was that no elastomer could stretch from zero to anything.......that being infinite-percentage elongation. I thought to vee-grove the outer part of the plank [imagine putting a chamfer on the outer plank edges before installation], thus creating a minimum seam width of at least equal to the plank movement. With this arrangement, the elastomer would only need a two-to-one elongation capability, which is referred to as 100% elongation, meaning going from one-eighth inch to one quarter inch.

A vee a quarter-inch wide, and about 3/16" deep, would easily accommodate a quarter-inch movement with a suitable elastomer.

At that time I was selling a two-component polysulfide rubber sealant which would easily stretch to accommodate this much movement without creating so much stress at the wood-sealant interface as to tear the sealant off the wood. 3M makes something similar, 5200, a one-component tough synthetic rubber adhesive sealant. that stretches easily, thus accommodates wood movement while maintaining a watertight seal. The moisture-diffusion-barrier-coating Epoxy Paint would go on top of sealant and wood-treated CPES, and that was the proposed coating system. The epoxy paint would of course develop many fine cracks when the sealant stretched, but it was still all glued to the sealant and the cracks would only go through the paint to the sealant. No water would leak in, and mechanical integrity would not be jeopardized. The fundamental design of the wood boat would be unchanged , and the fasteners would carry the load of the individual frames and stringers and planks as the original designer intended. My design modification only was intended to solve the water-leakage problem, not change any significant engineering characteristics.

I proposed this design to various boat-customers over the next year, and some thought it a great idea and did it. Others were highly offended that I would propose mechanical alteration of the vessel, even so minor as chamfering plank edges. Nevertheless, enough adventurous souls did buy into my [at that time] radical ideas, and do it, thinking as I did that it would work and solve this classic problem of all planked wood boats. I had created something based on engineering principles, designing something to work, using modern materials and modern engineering technology.

Over the following few years I heard from these customers from time to time, as they came back for various supplies, or just to visit and report on how things were working. The consistent result, which amazed every wood-boat-owner, was that this kind of "restoration", done as I have described, consistently resulted in a dry bilge and a mechanically stable hull regardless of whether the boat went in and out of the water or stayed in the water. This led me to write two short essays, "Wooden boats and wet wood" and "Wooden boats that go in and out of the water", which are part of the basic boat-related literature package sent to anyone who requests one [by reply email or phone to 1-800-234-0330, as we need a mailing address].

After some years and a few dozen instances of successful application it was clear to me that the solution I had invented really was a workable solution to the problems of planked wood boats, including some runabouts that were truly antiques and one commercial fishing boat I particularly recall, about a hundred years old [Fish & Game registration number 27].

I began to promote this more widely and in my literature as I learned more about marketing, and found over the following years that many of my old [earlier] customers had done pretty-much this sort of thing but in some cases actually bedded the outer layer of planks of a double-planked hull in my 2-part polysulfide rubber, behind the planks as well as in the seams. This, using today 5200, has come to be known as the Danenberg bottom, after Don Danenberg who [somehow] found out about me and CPES roughly 35 years ago. I [ever modest] never called my bottom-restoration method a Smith bottom until this subject came up in the middle-nineties on the Chris-Craft list, and [as I recall] someone got Don's method and mine mixed up and I wrote something to clarify our different approaches. I never heard of one that had a problem or needed a plank replaced, with either method. It is noteworthy that even though many of my customers are professionals such as Don, the vast majority are amateurs or hobbyists, and virtually all have, to my knowledge, been successful in restoring their boats by either method.

The procedure for doing this would be, first, to sand or heat-gun-and-scraper clean off the old paint. I'd prefer you not use chemical strippers, as some can leave a residue that screws up adhesion of subsequent things. Next, sand or scrape or cut a vee or chamfer where the seams are. Given a guide-strip, you could use a router or a small rotary saw. The idea is to have a gap everywhere the external seams are. It does not have to be a very large gap...and it does NOT go all the way through; it is a Vee. Friction between plank sides contributes to overall longitudinal strength of the boat, and is part of the original design of the boat.

Next, apply CPES to saturate the wood. This will take anywhere between 20 and 200 square feet per gallon, depending on the condition (porosity) of the wood. For example, the typical 16-foot runabout in decent condition takes a two-gallon kit to do the outer part of the hull below the waterline...very occasionally two 2-gallon kits of CPES. The stuff runs like water, and really soaks into abnormal porosity of old wood. Next, apply masking tape on each side of each seam, fill with 5200 and squeegee flush, and remove the tape before the sealant jells. This gives really neat seams, and finally a couple-three coats of my Barrier Coat, an epoxy paint I make that offers really high resistance to diffusion of moisture.

Whatever final bottom paint is desired may be applied over that. The same procedure, pretty-much, is done for lapstrake hulls, except that one does not vee-groove anything but instead puts a neat fillet of 5200 on the inside corner where each plank laps over the other on the outside. It usually isn't even noticeable.

Circa 2003 I was at an ACBS annual meeting and found myself sitting behind Chris Smith III on the deck of a boat touring around the Mackinac islands of northern Michigan. He told me the method used in boat construction by his grandfather Chris Smith (the first) was a few coats of enamel paint on the outside and a color-wash of the same paint, thinned 50-50 with turpentine, on the inside. What this gave was a moisture-diffusion-barrier on the outside and an inner coating that could "breathe" so water that got into the wood could easily evaporate off the inner surface.

That's why Chris-Craft boats had such longevity, and the Hackers and Hunters and Garwoods mostly rotted-away over the decades because both inside and outside were painted, thus trapping moisture in the encapsulated wood. I had in the early seventies independently invented the same thing to cure and prevent osmosis blistering on GRP hulls. See

So, here's the wood-boat restoration process in a nutshell: Put a moisture-diffusion barrier coating on top of CPES on the outside and CPES-alone on the inside; the wood can still "breathe", as CPES is permeable to water vapor.

Well, that's about all I can remember. Please feel free to ask more questions if you have them, and the 800-number previously given is good for ordering products also. Smith & Company products are available on-line from the Smith & Co. website as well as on the shelf in many stores around the country; the office staff can tell you if there is a store near you, and even provide handout information you can take into your local hardware, paint, building supply store or marina to induce them to stock product for you locally, should you wish to go that far. Here's that number again, 1-800-234-0330. I have, after all, learned something about marketing.

Steve Smith


© copyright Steve Smith, 1972 - 2021, All Rights Reserved.