You can enhance the appearance of an outdoor pine bench and protect it from the elements by finishing it with the appropriate coatings. Before you begin, understand a pair of critical points. Because pine is a hard wood, it tends to absorb stains at different rates. You must choose a light colour of stain, or you will end up with an unattractive splotchy finish. In addition, you must apply a particular topcoat, formulated to defend wood from moisture, or the pine will ultimately take on a grey, weathered appearance.
Clean the outdoor pine bench with a pressure washer equipped with a low-pressure tip. Wait a full day for the bench to dry.
Sand the pine bench, using a palm sander stocked with 220-grit sandpaper. Sand with the grain of the pine. Stop sanding when the surface appears dull.
Protect with painter's tape areas of the bench you do not want finished. Cover the area beneath the bench with plastic dust sheets. Place fabric dust sheets over the plastic ones to prevent stain drips from pooling.
Coat the outdoor pine bench with a light-coloured oil-based stain, using a paintbrush specifically engineered for applying oil-based paints. Wait 6 hours for the stain to dry.
Wash the brush with white spirit.
Coat the stained outdoor pine bench with varnish or polyurethane wood sealer, using the clean paintbrush. Wait 6 hours before sitting on the bench.
You may also finish the exterior pine bench with varnish or polyurethane wood sealer only.
Be sure to apply a varnish or polyurethane wood sealer finish after you stain, or the wood will eventually turn grey.