Linseed Oil Paint
Is this the most environmentally sound exterior finish for wood siding? This short video explains the process of finishing wood cladding with an all-solids (solvent-free) natural linseed oil paint. Linseed paint aficionados say this paint is more durable than any modern plastic resin paint, and a perfectly wholesome non-toxic substance. Here’s a catch: this paint is very expensive and evidently only available as an export from Sweden.
Here are some things you might want to know (about paint):
• The material commonly known as “oil-based paint” is perhaps more accurately described as solvent based. The solvent is petroleum distillate (aka white spirit / paint thinner). The binder is a synthetic resin called Alkyd, a petroleum derivative. Binder captures the pigment, so it doesn’t rinse or blow away. Solvent makes the mixture flow for ease of application.
• The binder in most water-based paints is acrylic, another synthetic resin derived from petroleum. Colorants (dyes, pigments) tend to be further examples of petroleum derivatives.
• Petroleum-derived synthetic resins (plastics) and colorants used in paint all tend to break down fairly quickly in daylight. Some natural resins (derived from plants) are superior in this regard. Mineral-based pigments can last forever.
• Long ago, paints were made with natural oils and mineral pigments. This went out of style as the paint industry evolved, in tandem with the petroleum / plastics industries.
• Plastic paints have one important advantage over natural oil paints: they dry much more quickly. They are also manufactured efficiently and are thus relatively inexpensive, and widely available.
A few details about Linseed Oil Paint:
• Linseed oil comes from the flax plant. There must be some reason that it’s called linseed rather than flaxseed oil. The best oil for paint comes from chilly northern climates. Sweden has a few brands of linseed paint that get exported throughout Europe. One of these is also imported to North America (Allback).
• Raw oil is very thin. It’s used to dilute linseed paint, e.g. for making primer that soaks deeply into bare wood.
• Raw oil takes a long time to dry. Oil cures as it turns from liquid to solid through oxidation. “Boiled” oil is heated and partially oxidized or cured in advance, so it dries faster.
• Hardware stores sell cans of “boiled linseed oil” but the oil is not in fact boiled or pre-oxidized. Instead it contains metallic drying agents that speed oxidation. There are small amounts of metal salts in most linseed paints. These are heavy metals (including cobalt, manganese and zirconium) that may be problematic for human health.
• Inexpensive linseed oil found in hardware stores contains impurities, including protein from the pressed flax. The impurities cause yellowing as the oil ages, and promote mildew or fungal growth. Linseed oil is notorious for causing mildew, but this may be mostly due to the poor quality hardware store product. Supposedly the very pure oil used in high quality Swedish linseed paint is quite resistant to yellowing and mildew and never becomes rancid. Zinc is also used to help fight mildew.
• It’s possible to apply linseed paint with no added solvent. The only solvent recommended by the Swedes is balsam turpentine, derived from tree resin. This material is also quite expensive. The Ottossan video series gives a few variations on painting techniques with or without solvent. A paint mixture thinned with turpentine will dry faster than one thinned with raw oil.
• Linseed paint adheres well to glass and metal. You can paint door and window hardware, nails, screws, trim and flashing.
• There’s a technique for staining wood with translucent linseed paint, which consists of a small amount of paint added to polymerized oil and turpentine. The result in the video is quite impressive.
Some guidelines about coatings for wood siding:
• Left uncoated, wood has a shorter lifespan. In some cases it could make sense to allow wood to weather unprotected, saving the trouble and expense of applying and maintaining a coating. Basically this would mean wood is the consumable, rather than a coating. At least this would allow easy disposal when the material is worn out—nothing for the landfill.
• When an effective protective coating is applied to a good quality wood siding with good detailing, the maintenance and durability issues for the siding are all about the coating, not the wood. So long as the coating is maintained, the wood should age very slowly. There are many centuries-old wood buildings that demonstrate this.
• To promote a long lifespan for the coating:
> Choose an opaque coating. Translucent coatings wear out at least a few years sooner.
> Choose a rough-sawn surface rather than a smooth planed surface.
> Choose a species of wood that’s at least “moderately durable”. North American Western Red Cedar is an example. Douglas Fir is classified as “somewhat durable”. (Reference)
> Choose all heartwood. The sapwood of any wood species can’t be considered durable.
> Apply the coating to all surfaces of the wood, which means the wood must be coated before installation. In the video, only the first of three coats is pre-applied.
Here’s what to expect for wood cladding with linseed paint:
• Over time, the paint loses sheen and color appears chalky. Adding a coat of polymerized oil every 5-10 years will restore color and sheen. By some reports, scraping is never needed, but that might not be true. Linseed paint forms micro-fractures but is not prone to peeling like a plastic paint. Unlike plastic resins, it does not trap moisture below the surface, which may tend to cause peeling and accelerated decay of the substrate.
• At year 7, apply a coat of boiled oil, wait a while and wipe off. In the Ottosson video this appears very easy to do.
• At year 14, apply another coat of pure linseed paint. Continue this cycled indefinitely.