Finishing Tips for a Good Boat

Amateur boatbuilders are able to create boats of remarkable quality, sometimes rivaling the finishes and detailing that comes out of the best of professional boat shops. Having built a few boats myself, I know that I have the experience and the skills to do work that is as good as many professionals, as long as I stick to fairly standard detailing and don't try anything very fancy.

I see work of similar or better standard from other amateurs but I also see some boats that are somewhat "agricultural" in appearance, with very rough finishes. Much of the problem is related to being unaware of, or ignoring, some fairly basic principles that will make a big difference to the finish without adding a lot of work.

It is worthwhile to do as good a finish on your new boat as you possibly can. The extra effort will pay off later in the price that you can get when you want to sell it and also in the level of pride that you will have in your boat when using it and when showing it off to others. You will put in a lot of time and money creating this boat, which may be wasted if your boat is sub-standard. The cost won't be any different if you follow these tips but the standard will improve considerably. So, be good to yourself and do a good job of the build.

Interior of Hout Bay 33 built by Sergey Fedorov in Russia. Note the lack of sharp corners on all of the woodwork, giving a soft appearance and a feeling of quality. 

In no special order, here are some tips to help you to create that high standard of finish.

1) Sand wood rather than coatings.
It is easier to get a smooth finish on wood than on epoxy, paint or varnish. Do yourself a favour and get the wood as smooth as possible before you apply your first coat of anything over it. This will give you a good foundation on which to build a nice standard of finish on the coatings that will come later.

2) Round off all external angles.
Here I am talking of the exposed corners of framing, trims, bulkheads etc., in fact every external angle in the boat.

There are multiple reasons for this. First, it looks much nicer, due to softening the appearance by removing the visual harshness of the sharp corners.

It allows the coatings to adhere better and be of more constant thickness so that they give maximum protection. Paint and epoxy is pulled away from hard edges by surface tension, leaving those edges with minimal cover and protection. Rounding them off allows the coating to spread itself around the corner, for a smoother finish.

It removes the sharp corners that can cause damage to human skin and bone if fallen against at sea, bumped when cleaning bilges or getting items out of lockers. Those sharp corners can also damage hoses, electrical wiring and other items of boat equipment due to constant movement and chafing at sea.

Rounding off the corners of framing like stringers, joinery cleats and trims is most easily done before installing them in the boat. Do so with a router fitted with a 6mm radius corner round bit on all edges that will be exposed on the completed boat. The simplest way to do this is to use a router table, with the router mounted below and the wood run through the cutter on top of the table. For corners of bulkheads, lockers etc. use a 10mm radius corner round bit on a router than can be run along the edge of the plywood.

Don't think that just because an edge is inaccessible it doesn't need to be rounded off. If you want that piece of timber to have long-term resistance to rot then it needs as much protection as you can give it. That means rounding the edges then coating properly with epoxy, even if you will never see it again.

This DS15 was built by Jim Foot in South Africa. This boat has a wet deck, with all spaces under the deck sealed in and never to be seen again. Nevertheless, all corners have been nicely rounded and all wood surfaces are smooth, ready to take the epoxy coatings. This ensures effective protection for the timber and improved durability of the boat, as well as higher resale value.

2) Smooth off all framing
There is no sensible reason to use timber that is rough on any exposed surfaces. Rough surfaces have many little exposed corners, so coatings will run away from those corners, just as they do from edges that haven't been rounded off. You also increase the risk damage to skin, hoses etc. as described above.

You can buy wood that is planed all round, in the sizes that you need them to go into the boat. However, that is the expensive way to build a boat. It is normally cheaper to buy your timber in wider planks then cut them down to size yourself. Buy a table saw to do this job, it is a valuable piece of equipment for a boatbuilding project. Fit a carbide-tipped blade with 40-50 teeth. That will rip the wood quickly and leave surfaces that need minimal or no sanding.

The saw doesn't have to be a big one, costing loads of money and needing lots of space in the workshop. I built the CW975 "Concept Won" using a skill saw mounted under a portable saw table. When not needed I could pack it away while I did other work. At the other extreme, I built the Didi 38 "Black Cat" using a very nice radial arm saw.

For large boat projects it is worthwhile to buy a bench belt sander with 4" wide belt, which will allow you to run framing over the top of it to remove any roughness if needed. It will also allow you to sand smaller items to a nice finish before fitting them into the boat and to fine-tune the ends of smaller pieces of framing that need to meet neatly at intersections. Attention to these smaller details will make a big difference to the professional appearance of your boat.

Don't sand any frame surfaces that will be glued, rather leave those surfaces slightly rough or clean-cut. Sanding will block the pores of the wood, reducing penetration of whatever glue you use, thereby weakening the bond.

3) Take care with fillets.
Take care and time to make your epoxy fillets constant in radius and shape. A fillet that sags is not pretty and is also weaker than one with a smooth curve that feathers out against the two surfaces that it connects. That can sometimes be difficult to achieve because a high density fillet formed with epoxy, cabosil and wood flour cures very hard and is hard to sand. A little too wet a mix and it will sag, whereas too dry a mix drags when you run your filleting tool over it. A low density fillet is easier to sand out imperfections after it has cured. It is not as strong, so must be larger for equal strength. If you have an ugly high density fillet and can't rework it to look better then add a low density fillet over it to hide it.

4) Sand between coats.
Sand your wood surfaces before applying the first coat of epoxy, paint or varnish. After that blow off all dust with compressed air or vacuum it away. If you don't, you will be rolling dust and wood fibres over the surface when you work. They will be set into the coating and stay there, creating bumps and texture on the surface. The only way to remove this will be by sanding the coating before applying the next coat.

Some of the surface fibres of the wood will often stand up when the first coat is applied. Don't ignore this and paint over it, this will require more coats to cover it. Instead, it will need only a light sanding to smooth the texture before applying the next coat.

Sand lightly between coats if more time has passed than the over-coating time recommended by the manufacturer. For epoxy this is normally 12 hours, for paint it varies. It is also worthwhile to do a chemical wipe with acetone or mineral spirits to remove any surface contamination that may have come from skin contact (body oil in the shape of hand prints may be the most common form of inter-layer failure in painting of cars, boats and furniture) or other sources of contamination. A diesel generator running nearby can ruin your paint job due to oils in the exhaust gasses raining down on everything downwind of it.

5) Feather glass edges
If the boat that you are building has glass-taped joints or areas that are reinforced with fibreglass, sand the edges of the glass to a feather edge then apply a final coat of resin to fill the weave and seal any exposed fibres. Fair the glass surfaces where needed with epoxy fairing compound, as well as along all edges. The fairing should be no thicker than needed to fill any hollows, with minimal thickness over the humps. Sand it smooth to make the glass reinforcement disappear into the surface once it has been painted.

This is the chine of the Didi 950 being built by Mike Vermeersch. It has 2 layers of glass tape, which has been sanded to feather edges, faired and sanded smooth.

The finished chine with epoxy primer, ready for paint.

Achieving a great finish on your new boat should be a major goal. You will experience much more pride and pleasure from a boat that looks really nice than one that is very obviously an amateur build due to inferior surfaces. Challenge yourself to work to the highest standard of finish that you can. You will surprise yourself.

To view our designs, go to our main website or our mobile website.

More on Torturing Plywood

Following on my previous post, about bending plywood into boat hull shapes, here are two photos to show what can be done. These photos are from Ian Allen, who built his Cape Cutter 19 from plywood in New Zealand. The first photo shows a bottom panel from the bow area, clamped and braced in place on the hull framing. Ian is using sliding bar clamps at the edges, backed up by wood temporary framing and Spanish windlasses to add pressure in the middle of the panel where clamps don't reach. The panel was left in place until it relaxed into the shape.

The panel clamped in place. Note the wedges under the wood framing, to apply pressure in the middle of the sheet where clamps don't reach. The panel is left in place to stretch and relax into the required shape. Wetting or steaming the panel will speed up the process.

The second photo shows the panel after release from the hull framing.


Panel after removal from hull framing. Now it will be much easier to glue and clamp to the framing.

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Torturing Plywood in Boatbuilding

Plywood is a great engineering material. Man has taken natures wonder engineering material, wood, and improved on the structural characteristics for use in large sheets. In it's natural form you can't cut wood into 1/4" (6mm) thickness and expect it to remain stable and strong in sizes more than a few inches in width when measured across the grain. It will split, it will twist and it will warp out of the nice flat form that you originally cut. It will be strong along the grain but relatively weak across the grain, which must be taken into account in any structural application.

When that same wood species is cut into thin veneers and they are glued together with the grain direction of adjacent layers oriented at 90 degrees to each other, each layer strengthens those on each side of it in it's weakest direction. The resulting material loses it's linear strength characteristic and strength becomes relatively similar in all directions. The material also becomes much more stable, much less likely to twist, warp and crack. This brings great benefits for designers using this material but it also introduces limitations that the designer or builder must work around. In short, the benefit of plywood is that it is flat and the drawback of plywood is also that it is flat.

Of course the wood species, the quality and number of veneers and the quality of the glue and manufacture all affect the strength and stiffness of the sheet. As a simple rule of thumb, the more veneers, the stiffer and more stable the sheet will be, when comparing sheets of the same wood species. When comparing sheets of different species, the more dense (heavier) specie will likely be the stronger one. Here is an excellent article on plywood quality.

Plywood structure. Plywood must always have an odd number of veneers, so that the surface veneers on both faces run in the same direction. This is called balanced construction and adds stability to the sheet. If it has an even number of veneers then the surface grains will run at 90 degrees to each other and result in tensions within the sheet that will cause it to warp.

Ever since plywood was first invented, man has been figuring ways to distort it into shapes other than the flat sheets in which it is formed. There are various ways to do this and understanding the nature of the material helps to figure the best way to deform it to suit our needs.

1) Wood can be stretched, within limits. When we bend it, the outer surface of the bend stretches and the inner surface of the bend is compressed. The fibres in the middle are on the neutral axis and are neither stretched nor compressed. The fact that it can be stretched means that it can be deformed in ways that are not necessarily conical or cylindrical in nature, the principles of developable surfaces normally used when designing curved surfaces for sheet materials. It can be somewhat tortured into shapes that mathematics says can't be done. But, push it too far and it will fracture, proving that the mathematics may have been right after all.

2) Wood can be softened and made more pliable if it is wet, especially if it is also heated at the same time. This is the principle used when making steam-bent frames in traditional boatbuilding, softening the wood strip in a steam box. A stiff piece of wood becomes quite soft and malleable when hot and wet, so it can be bent to shape and will hold that shape when it dries and cools. This can also be done with plywood but how do you do it with a big piece that won't fit into a steam box? I have made plywood sheets more flexible by laying them on wet grass, spraying water over the sheets then covering with black plastic and leaving in the sun for a few hours. Others have had similar success using a steam wallpaper stripper on the outside surface while gradually pulling the sheet in with clamps and other mechanical tools. If you have a sheet clamped in place and need to coax more bend into it but don't have a wallpaper stripper, spray water onto both sides of the sheet then cover with black plastic to absorb heat. Even without heat, you can wet the sheet, keep it wet overnight with hessian bags or similar, then find that it is easier to bend in the morning.

Steam wallpaper stripper can be used to make plywood more pliable

3) Wood can be bent by making saw-cuts (kerfs) across the piece on the inside of the curve. This reduces the compression loads on the inside surface and effectively moves the neutral axis locally to the mid-point of the thickness that is left after the kerf is cut. If the kerf is half-way through the wood then the bend characteristics become more like a piece of half the thickness than one of the full thickness. The more kerfs that are cut the more that the piece is softened and the smoother the curve that results. This works with plywood also but must be judiciously done. The kerfs shouldn't be cut deeper than half-way through the plywood, or the structure of the plywood will be destroyed. If you cut them so deep that only the surface veneer remains, you will almost certainly fracture that surface veneer along the kerf line. The kerfs should also not be excessively wide because they will have to be filled with epoxy to regain the lost strength after the panel is glued in place. The width of a circular saw blade or narrow router bit, so about 3mm, is a good kerf width. It gives enough width to allow the sheet to bend without the edges of the cuts closing so much that they can't be filled.

4) It is easier to deform short sheets a small amount than long sheets a large amount. If you have a panel that is 20ft long and has considerable twist to it, it will be difficult to pull the twist into that long panel if it is all in one piece. It will be easier to get that twist if the panel is fitted in three 8ft lengths that are glued to each other on the hull framing. It will be even easier if fitted in six 4ft lengths but that is only required if the twist is extreme. As example, for the Cape Cutter 19 or Cape Henry 21, with lots of twist in the bow and not much twist aft, the panel would be best fitted as 4ft lengths in the bow, changing to 8ft lengths further aft.

5) You can't easily deform a sheet so that one edge has convex curvature (curved outward) and the opposite edge has concave curvature (curved inward). The stresses in the sheet will likely damage it. The builder of the first Didi Mini to be skinned emailed me to say that he had found it impossible to skin the side panel forward of the mast; the sheet simply could not be forced into the required shape. It has convex curvature at the deck to add reserve buoyancy and concave curvature at the bottom in the form of a hollow bow waterline, to improve wave penetration and increase speed. I told him to cut the sheet vertically so that it was two sheets 4ft long instead of one sheet 8ft long. He emailed back that the problem disappeared and he could easily skin that area.

6) Narrow panels are easier to twist than wide ones. If you have a long and narrow panel that is twisted and another that is the same length and twist but 5x the width, the narrow one can possibly be fitted in one length but the wide one will need to be broken down into shorter lengths to make it easier to fit. My radius chine designs generally have considerable twist in the bottom panel toward the bow. There is no problem twisting this panel because it is narrow in that area.

7) Plywood will take on a set if it is distorted and held in the distorted shape for a few hours. If it won't pull all the way into the shape that you need, don't force it too far, you will break it. Pull it in with clamps, levers and Spanish windlass as far as it will go, then leave it. If you were to unclamp it a day later you would find that it doesn't want to lie flat because it will have taken on some of the bend due to stretching of the wood. Instead, go back 2 or 3 times in 24 hours and pull it in some more. Eventually you should be able to pull it all the way to where you want it.

Ian Allen, building a Cape Cutter 19, used clamps and levers with
Spanish windlass to pull in the bottom panels in the bow. He used
an 8ft length and would have found it easier in two 4ft lengths.

If you are building a boat that has a skin panel that has a large amount of twist that is mostly concentrated in one area then you are almost certain to need one or more of these methods to get the plywood to conform to the hull shape. You can combine various methods from the list, to supercharge the process. Softening with water/heat or steam, combined with kerfs and fitting narrow sheets will allow plywood to take on considerable twist. Once you have managed to pull that panel and it's partner on the other side to the form that you need and they will hold most of the twist when released, then it is time to glue it into place. Don't consider getting one side done and glued in place before starting to twist the other side, you will seriously limit your options for clamping any edges where the two panels meet. You will also load the framework on one side and possibly pull it out of alignment so that the completed boat is asymmetrical. It is best to prepare the one side then set it aside while you prepare the other side, then glue both on in parallel.

To see our boat designs, go to our main website or our mobile website.

Fibreglass Boats from Plywood Plans

There are many, many options available to build a boat from plywood plans. If you want to build a small fiberglass boat as an amateur, the choices are much more limited.

There are ways to go about it and occasionally someone does. Phil Coveny bought plans for our smallest design, the 8ft Dixi Dinghy. It is a plywood 3:1 dinghy intended as a yacht tender or a child's first boat. It just happens to also be a fun little boat for older children to sail (read that as Dad or Grandpa).

Phil wanted to build his boat from sandwich fiberglass for various reasons that included an available supply of the needed materials. He built her with perforated 9mm foam with a density of 80kg/cu.m, glassed both sides with 9oz E-glass fabric in polyester resin. You can replace the polyester resin with epoxy for a more durable boat but at higher cost. He worked with the foam panels in much the same way as he would have with plywood except that, instead of doing stitch-&-tape joints, he used a hot-glue gun to do joints, then filled the joints with polyester putty before glassing over the whole boat, inside and -out.

Bare foam hull with joints glued then filled with epoxy putty

Closer view of the perforated foam, with glued and puttied joints.



Hull has been glassed and seats are being bonded in.

Interior view of finished boat, with wood rubrail and rubber bumper.

Bow view of completed boat.

Phil sailing his new tender. This rig is from another boat that Phil built, a bit smaller than the Dixi rig.

Phil's boat came out at 65lb (29.5kg), somewhat heavier than he had hoped, primarily because he didn't use a squeegee to remove excess resin from the laminate. Removing excess resin binds the glass fabric closer to the foam, reduces resin usage, reduces weight, increases strength and improves the finish. If you don't have prior experience with glassing then it is worthwhile to practice the technique before you start to glass the hull.

If your motivation to build a small sandwich fiberglass boat is to build it lighter than with plywood, you are unlikely to achieve what you want. Modern okoume plywood produces a very light boat (about 48lb for the Dixi Dinghy) and the foam sandwich fiberglass boat will need very light skins (max 6oz for the Dixi Dinghy) to have similar weight to plywood.

Our other plywood dinghies can also be built in the way shown in this post but they are only detailed for plywood construction.

Visit our website to see our dinghy designs or our full range.