Boiled and raw linseed oil

Overview

Linseed oil is pressed from flax seed and is a drying oil: it hardens by reacting with oxygen, not by evaporating. It soaks into the wood and leaves a thin, soft, warm-looking surface with almost no film. Woodworkers pick it for traditional furniture, tool handles and period work where a low-sheen, repairable finish matters more than protection. "Boiled" oil is not usually boiled; the Wikipedia article describes it as raw oil with metallic driers (and sometimes stand oil) added, and Tried & True's FAQ makes the same claim about most commercial products, naming cobalt and manganese as the driers.

Pros

• Very cheap, available everywhere, and easy to wipe on.
• Penetrates and enriches grain without building a film that can chip or peel.
• Spot repair is simple: clean and add more oil.
• Boiled oil dries in roughly a day per thin coat; raw oil takes far longer.

Cons

• Thin protection. The USDA Wood Handbook notes oil finishes are easy to apply and maintain but soil more easily than film finishes.
• Poor outdoors: the Wood Handbook says linseed and tung oils feed mildew and are suited to indoor use away from water or high humidity.
• Raw linseed oil may stay tacky for weeks; Wikipedia says it does not cure well or quickly enough to be treated as a drying oil on its own.
• Yellows and darkens with age; low water, heat and chemical resistance.
• Over-application leaves a gummy surface that can take weeks to harden.

Suitability

Good on open-grained and figured hardwoods where warmth is the goal: White oak, Black walnut, European walnut, Black cherry. It suits low-wear furniture, tool handles, and reclaimed boards. Avoid it on very oily tropical woods such as Teak and East Indian rosewood: Wikipedia's wood-finishing article notes that extractives in rosewoods retard the curing of some coatings, so wipe with solvent and test a scrap first. Not for kitchen counters or exterior work.

Coats and application

Sand to 180 to 220 and remove dust (Sanding schedule). Flood the surface with a rag or brush, let it soak 15 to 30 minutes, then wipe off every trace of excess; oil left on the surface is what stays sticky. Apply 2 to 4 coats, one a day for boiled oil. Liberon's instructions are a good model: dilute the first few coats with turpentine (up to 30%), rub in, wipe off the excess after about 20 minutes, repeat at 24-hour intervals, then let the final coat harden for a few days; its label warns that unremoved excess gives a sticky surface that can be wiped with white spirit. Leave a week or more before heavy use. The Wood Handbook suggests two or more oil coats followed by paste wax (Paste wax). See Wipe-on application for technique.

Compatibility

Cure fully first, then it will take wax. Oil-based varnish and polyurethane can go over well-cured linseed, but adhesion is a risk if it is not dry; a test patch is wise. Do not put water-based finishes over a fresh oil surface. Dewaxed shellac (Dewaxed shellac) is the usual barrier coat. Oil can darken light stains and drive pigment into end grain (Finishing end grain). Compare Polymerized linseed oil (stand oil) for a faster, harder oil.

Care and repair

Refresh with a thin coat of oil or a wax every year or two on used surfaces. Rings and scuffs can be rubbed with 0000 steel wool and re-oiled. Strip with mineral spirits or sand back. Do not mix mineral-oil furniture polish into the routine; it stays wet and blocks later coats.

Safety and environment

Oily rags can ignite by themselves. The USDA Wood Handbook says rags wet with oil-based finish are particularly susceptible and should be immersed in water and sealed in a water-filled airtight metal container immediately after use. Allbäck says both raw and boiled linseed oil can self-ignite in porous material and rags should always be soaked in water. Klean-Strip's SDS gives a different method: spread rags flat outdoors for 24 hours above 40 F, or wash them thoroughly with detergent and water. Follow your local rules; see Oily rag safety.

Commercial boiled oil contains metal driers; the Klean-Strip SDS names cobalt and manganese. No general food-contact certification applies, and this entry makes no food-safe claim; see Food-safe finishes. Wear gloves, ventilate, and keep away from ignition (the SDS gives a flash point near 99 C).

Related

• Polymerized linseed oil (stand oil): heat-bodied, no driers, harder and more reliable.
• Tung oil: harder, more water-resistant drying oil.
• Danish oil: oil-varnish blend that cures harder.
• Paste wax: the usual topcoat.
• Oily rag safety: disposal in detail.
• Linseed oil paint: same binder, pigmented.