Linseed oil impregnation
Why should anyone impregnate a boat with any kind of oil?
The biggest problem of wooden boats is that wood is a living material and it expands and contracts with the moisture. To minimize this, the boatbuilder has to find means to keep the wood as stable as possible.
To diminish the contracting and expanding of the wood, boatbuilder replaces water with oil and unlike water, the oil does not dry out and the timber remains more stable. Even if all of the wood material is not impregnated, the oily surface of the wood considerably slows down the water movements inside the wood, keeping water content down and dimensional changes at a minimum.
Here is the whole point in impregnating the boat: Since nothing can be used as an absolutely reliable barrier to stop water entering the wood, the wood should be filled with something else that repels water. That is why different kinds of oils from whale oil to soy and linseed oil and tar have been used to impregnate old wooden boats. I'm not saying that the oils are "the newest word of the modern science" what comes to modern boatbuilding, but that approach has worked for at least a few thousands of years and it still does. Oils don't work everywhere, but on traditional classic boatbuilding they offer the best, most economical and environmental approach.
One word of caution though: The use of oxidizing nature oils (linseed, hemp oil, whale oil, seal tran etc.) and on the other hand, pine tar and turpentine have been used for thousands of years in the Scandinavia and Northern Europe. They have been used with the local boatbuilding timbers pine, spruce and oak. Linseed oil impregnation has been used very successfully in Scandinavia with mahogany to keep the water content down and it definitely helps minimizing damages when wood freezes on wintertime. The world is definitely larger and the materials endless, so I would be quite careful to use the same ideas in warmer climates, different oils and woods. The people around you should know better; just find a boat builder who is old enough to have forgotten how to spell "polyester resin" and you should be on the right tracks. Or then not.
The whole idea of impregnating a boat is in getting the oil as deep in the wood as possible. It is said that no surface treatment will soak any deeper then a few cells in the wood. Sadly, that's very true, but linseed oil impregnation isn't a surface treatment. Think of a leaky diesel engine in your boat: diesel oil seeps through the planking in an alarming speed. Just as well thin linseed oil can be soaked through the planking, if necessary.
New rowboats can be impregnated through the board so that you pour a few gallons of oil in the boat and spread the oil to the insides as long as it starts to come through the planking on the outside. Then you still put a couple of dozen coats on the outside, let the oil set for a while and varnish, paint or usually around here, tar the boat. On bigger boats it's not so easy to get the oil through the board but you still apply the oil as long as the wood soaks. For a new rowboat that can be 2-4 days, for a big sailboat you could have to apply a coat every now and then for a few weeks.
We impregnated a 13-metre traditional Finnish lapstrake gaffer a few years ago. Records show that the boat got 50-60 coats of oil all over. It's a quick job doing it once, but this took a while. Later we drilled a drain hole on the bottom and found out that the lowest board, about 30 mm (1 1/4") thick was impregnated with oil and tar all the way through.
A problem with linseed oil is that you should allow it to dry for at least week and a half before even thinking of painting. With some paints and varnishes you are able to start the paint job straight over a freshly oiled surface wet-to-wet, but as this doesn't work with many paints I wouldn't encourage doing it that way. If you are not ready to wait a couple of weeks, you'll have to use boiled linseed oil to seal the surface before painting. Even with boiled linseed oil you still have the problem of turpentine trying to evaporate through the finish, which may cause the paint lifting off.
Again, there are some known boatbuilders, who start painting or varnishing straight to a wet oiled surface. Some paints work, some don't. Usually a natural oil based paint or varnish should be safe, but try it first somewhere, some paints are a complete disaster over undried oil.
Some urethane and practically all of the polyurethane paints don't stick to the impregnated surface. Oil based traditional paints and oil based spar varnishes are the safest. If the paint or varnish is linseed oil - tung oil based, it will dry as an integral part of the impregnation. Other paints don't necessarily mix as well although there are many good exceptions.
You have as many recipes as for distilling moonshine.
Straight linseed oil itself is yummy to mold and mildew and if you use it straight the surface will get dirty and spotty in a short time. Pine turpentine helps a bit to prevent mildew, but not too much if you live in a hot and humid area.
The best way to avoid mould is to add a dash of a clear wood
preservative to the mixture. Preservative should be just white spirits
with a rot toxic (as a zinc or copper naphtenate, tolyle fluanide
etc.) added on. 10-15% will be enough. If you are going to impregnate
the whole boat, even more can be used.
In Sweden they tend to use much less turps and wood preservative for the basic recipe (with usually 2/3 or more linseed oil) and for tarred boats 1/3 oil, 1/3 turps and 1/3 tar.
If you are going to paint or varnish the boat, as usual, leave the tar out altogether. Some people use a dash of tar under varnish for a beautiful tint in the wood but as it doesn't work well with all varnishes it's safest to leave tar out unless you know that the combination actually works.
If the wood is soft (like pine, spruce etc.) you can use more oil and less turps because it soaks in anyway. With oak or mahogany you have to dilute the oil a bit more. If it's cold the mixture has to be thinner. Basically you try to get as much linseed in as possible and dilute it only as much as is necessary to get it there. If the surface stops absorbing oil, you can use a few layers of straight turpentine to open the surface and go on with oil.
Boiled linseed oil is less prone to catch spots so you can substitute that for raw linseed oil, at least for the last applications. It doesn't penetrate as deeply to the wood as raw. It also creates some sort of a surface when straight linseed oil just soaks in. Hence you can use boiled linseed oil or even thinned tung oil varnish for the last applications.
Adding a drier (japan drier, siccative) helps against mould because the dessicants usually are toxic metal salts and the germs don't like that. However when using a siccative the surface may set too early before the oil soaks in properly. I've done well without it treating a few dozen boats but your mileage may vary.
Linseed oil on plywood
Plywood isn't a particularly good candidate for linseed oil impregnation, since the oil doesn't go any further than to the outermost ply. Benefits of a combination of oil and a soft oil-based paint is to let a massive piece of wood live and breathe, but plywood can be finished with a tougher paint system.
If the boat stays out of water most of the time, you will be OK by using oil and an oil-based enamel paint. Pay special attention to the plywood edges: use the oil liberally and apply several layers of 50/50 thinned paint to them before starting with the whole hull.
I'm a bloody traditionalist anyway, but plywood is about the only place where I would suggest using epoxy as a finish. Thousands of plywood boats have been built without it, but it definitely has its advantages on plywood.
© Pekka Huhta