Sihistin - Finishing

Tar, what it is and how it is made?

Pekka Huhta — Mon, 01 Oct 2007 14:04:54 +0000

"Stockholm tar", pine tar that is, has been made for thousands of years. The name Stockholm Tar has a centuries old history. Tar was mainly made in Finland (which was a part of Sweden those days) but as it was all sold through Stockholm, the name came and stayed. The poor Swedes had burned most of their forests with their iron industry they came over to the periphery (Finland) and bought our forests in form of tar.

Tar and pine turpentine both come from the pine tree, but in these days they come from totally different process. Pine turpentine comes out as a by-product for paper industry and tar is burned in a tar pit. In the old times it was a different story.

Originally tar was made by damaging the trees over 2-3 years. The lowest part of a pine was barked and only a hand's width of bark was left to keep the tree alive. The tree created a large amount of pitch, but most of it is inside the wood so just collecting the pitch wouldn't have been too productive. After several years of maltreating the tree it was felled. Also the stump was used, since it is the pitchiest part of the tree. Today the damaging is usually not done; they just get less tar for a certain amount of wood. And the forests are full of pitchy stumps to be used for tar after the trees have been harvested.

The wood was chopped to smaller bits and stacked to a tar pit. A tar pit is originally a cone dug out in the ground and covered with watertight (actually tartight) bottom with a hole in the lowest part to collect the tar. A big tar pit could have been 20 meters in diameter (22 yards). I found a site with original pictures about building a tar pit at http://pattersonhistoryproject.com/www.pattersonhistoryproject.com/Tar_K.... The pit was covered with peat and earth and set on fire so that it would burn with the minimum amount of air. If there was too much air all the tar would burn away, with less air it's like distilling solid firewood. Today usually a metal "tar pit" is used, since the airflow is easier to control. A big pit from the old ages could burn for two weeks and produce 5-10000 gallons of tar and large amounts of charcoal, nowdays the amounts are much smaller.

The first fraction coming out of the tar pit is called "tar piss". It contains mostly water and is useless. Next fractions are called "black tar piss" and it's generally "old-fashioned" turpentine, it's just darker as the industrial product today. After that you get tar and finally 'pitch', tar of a consistency of road bitumen.

So tar is in fact liquid smoke, if you think of that. Pitch and turpentine is what keeps a growing tree from rotting alive, so tar is just the "natural rot preservatives" distilled out of the tree. It works brilliantly with pine, which is not an extremely rot resistant tree by its nature.

A fresh product

Tar is a sort of a fresh product. I use about a gallon and a half every year for regular maintenance of three small boats and occasional projects. If it's stored in a plastic container, the lightest fractions go straight through the plastic and your tar can looks like a raisin in the springtime.

The difference on fresh and old tar really can be seen on light wood: fresh tar brings an light amber colour after a few applications and several years old tar would make the surface very dark with the same finish.

Tar can be used old, but it's not the same. My boat was tarred for years with tar that was dug out of 10-year-old tar pit. The tar was full of sand and consistency was closer to pitch but it protected the surface anyway. Ugly it was, but the wood was safe.

Old tar can be thinned down a bit with pine turpentine (some prefer ethanol as a thinner) and linseed oil to get it soak to the wood as fresh tar. And tar is always applied hot, regardless of whether you have thinned it or not.

Always apply it hot

Tar should be applied hot to allow it to soak into wood better. It's fairly thick as is and at least here the weather is still pretty cold when it's time to tar the boats. If you apply it cold, it'll just form a useless surface on the wood. You want to get it inside the wood, or at least the majority of it. Adding linseed oil helps on soaking in and as a bonus the tar doesn't get soft and sticky in the sun. On the other hand too much oil will destroy the best feature of tar: self-repairability. As tar softens in the sun it melts and fixes any cracks or small dents in the coating.

How to heat tar safely? Never leave the tar boiling unattended. If you take it from the stove in time, before it's too hot you are safe. If you allow it to go on a hard, rolling boil, you can get a four feet fire from the kettle and that's a handful to extinguish.

Tar has to come only to a very slight boil. When light brown bubbles start to form on the surface boil it still a few minutes but not more. When it starts to smoke it's a bit too hot. Start applying and when the brush starts feeling sticky against the boat, get back to your stove.

I've used open fire collected from the driftwood on the shore, gas cooker, paraffine cooker (which looked like a giant 2-flame oil lamp), kerosene blowtorch, even a disposable barbeque grill. They all worked although the open fire was a bit too much trouble because the high flames tended to light the tar a bit too often. With other methods I haven't had more than occassional problems.

Where to find tar?

In Finland that's hardly a problem, any hardware store will do. At the Suomenlinna shipyard it is bought by the barrel for the old Baltic Traders. But around the world it's a totally different story. Classic boat mail-order houses have it in ridiculous 1-liter jars with a sad price tag on top. Some people suggested a horse chandlery, since tar is used for treating hoofs.

Hard to tell where they make the tar these days. I have seen tar, which was made with some sort of pyrolysis, not by burning the wood itself. It was horrid stuff, and it smelled like old burnt newspapers. There are not that many places where they are willing to see all the trouble for making proper tar.

As an addition to the "odds and curiosities" list I just had a dish of tar ice cream at the local restaurant. We Finns still have a funny yen for tar, we have tar candies, different kind of tar-flavoured liquors, tar shampoo, and I've even eaten tar-marinated herring. And tarring my boat on the boatyard seems to bring dozens of people there, just smelling the air and smiling.

Linseed oil impregnation

Pekka Huhta — Mon, 01 Oct 2007 10:38:49 +0000

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.

Technique

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 Finland the basic recipe tends to be close to 1/3 of raw linseed, 1/3 pine turpentine and 1/3 of clear wood preservative for killing rot and mildew. If it's being used for a boat that's supposed to be tarred (or inside of an open boat with no other surface treatment) one would use less wood preservative (or none at all) and add more and more tar as the impregnation goes on until the tar content would be perhaps 1/4-1/3. Yes, it gets black in years and yes, you should use less tar to get a lighter surface. A tenth or two of tar in a small boat makes a difference. A spoonful in a quart is sissy.

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.

The use of epoxy in wooden boat building

Pekka Huhta — Mon, 01 Oct 2007 09:44:35 +0000

To build a new wooden boat, you can use epoxy for ease of use and performance. Or then use the traditional methods which don't require using epoxy at all. If one has thick enough and good enough lumber I can't see a reason to use epoxy on a small boat. Everything can be done without glue, and if the boat is done without glue it's always easier to repair or replace a damaged piece when you don't have to fight with glued seams.That was the idea with classics: when something was broken you could pull the broken piece out and replace it. It just doesn't do with epoxied structures.

Of course using epoxy is not out of the question. It just shouldn't be the default or "essential boatbuilding material". There is no actual need to use epoxy on the maintenance of a basically sound wooden boat which has been built without it. Restoring or bigger repairs are always a different case.

In restoring a classic wooden boat, you should use traditional materials in a traditional manner. This is even more important if you are doing major reparations or a complete rebuild. New material and new building techniques will leave nothing of the original, new material with original techniques will fit in much better. The only exeptions to this should be allowed if the desired strenght of the structure cannot be achieved without epoxygluing.

Using epoxy is sometimes handy and may save you from taking too much of the original apart. There are places that can be quite easily repaired with epoxy. Doing things "the traditional way" can require taking down much more of the original structures. If something is rotten, it is absolutely wrong to try to substitute the rotten wood with epoxy or a plastic putty. The only right way is to repair it with new wood.

Oils and epoxy shouldn't be used together. If something is to be glued with epoxy, oil impregnation should be done afterwards, not before gluing. Epoxy might not bond properly on a previously oil-treated surface, so in case you have to use epoxy, clean the surface with acetone or such.

When doing a repair on a classic boat you should avoid using epoxy as a sealer. Excluding the fact that on a real classic you will ruin some of its historical value, you are more likely creating just pockets where the water gets in but can't get out. Epoxy encapsulation works, but it should be used only if the encapsulation is done while building the boat. It is almost impossible to encapsulate a ready structure reliably and that will result to a situation where there is water trapped inside the wood and it can't evaporate. And that means that the wet wood freezes at the wintertime and it is destroyed in a few years.

In restoration work epoxy is a great glue, but it shouldn't be used to bond together things that should be able to move. The original designer has designed the boat accepting the fact that the structure will move some, and depending on it. Now when you stiffen part of the structure with epoxy, something else will break.

Wood movement, UV light, scratchs and knocks may break the epoxy surface. So, it's not as "service-free" as one would think. You have to keep the surface in perfect condition to keep the water out.

Covering the whole boat with epoxy and/or fiberglass should be reserved to the most hopeless cases of rotting fish traps. The trick which may extend the life of a wreck for a few years may as well turn a reasonably sound boat into an un-repairable and quite worthless junk that has very little value. After fiberglassing the hull there is not much left to do. The process can't be reversed.

Otherwise there is no general rules in salvaging those absolutely hopeless cases. You can use ethylene glycol, fiberglass, anything you can think of, but only if you are absolutely sure that there is no other options.

I saw a great little motorboat last year, and found an article about it in a Finnish wooden boat magazine. The boat was very thoroughly restored: in fact only thing there was left of the old construction was the keel, five planks and a few parts of the small cabin. It just shows that some poor fool can find a boat where everyone else sees a just hopeless wreck.

I wouldn't have put so much effort on such a wreck. But the owner is more than happy with it. Don't consider your wooden boat a wreck before asking someone who knows.