If you want to give your outdoor woodwork and furniture a protective finish but keep its surface free of reflective films, boiled linseed oil is just what you need. It's the main ingredient in many varnishes, but without the addition of an alkyd or polyurethane resin it penetrates deeply into the grain, where it gradually hardens to seal out moisture while leaving the surface looking rich and natural. It's easy to apply, but the wood must be dry and clean.
Derived From Flax Seeds
Both linseed and flax oil come from seeds of the flax plant -- Linum usitatissimum -- but they are processed differently. Whereas flax oil is produced by cold-pressing the seeds to produce an edible product, linseed oil is produced by boiling the seeds and extracting the oil with solvents, which results in an inedible product that has been used for centuries as a paint binder and wood finish. Oxidizing boiled linseed oil and processing it with thinners -- a treatment referred to as double-boiling -- enhances its drying and hardening characteristics; this is the product that usually bears the "boiled linseed oil" label.
Prepare the Wood
Clean the wood with soap and water to remove mold, dirt and anything else the oil could lock into the grain.
Wait for dry weather to finish your outdoor woodwork -- applying oil to wet wood can cause the oil to bead, or it could trap moisture in the grain and cause warping or deterioration.
Sand the wood with 100- or 120-grit sandpaper to remove any surface discoloration caused by sunlight. Sanding also opens the wood grain and helps the oil penetrate deeper.
Apply the Oil
Fill a bucket with linseed oil and apply it liberally to the wood with a paintbrush. More is better than less -- you can always use a rag to remove what doesn't penetrate after waiting about 30 minutes.
Let the oil dry overnight, and then lightly scuff the surface of the wood with 120-grit sandpaper to knock down the grain raised by the oil. Sand with the grain to avoid leaving visible scratches.
Apply a second coat of oil, using a paintbrush or wiping it on with a rag. Apply a smaller amount of oil, because the grain is already sealed and less will penetrate.