I've got good news for many of you. Your first finish option is 100% cost and labor free - you do absolutely nothing. Because of the remarkable inherent properties of cedar (its resistance to rot, decay, and insects), many decide to leave their cedar furniture unfinished.
Both Western Red Cedar and Northern White Cedar are perfectly at home outdoors. Building codes across North America recognize cedar as a durable wood for exterior applications. Downed trees, over 100 years old, have yielded usable wood. Think of that! With proper care, your outdoor furniture lasting 100 years!
Cedar's secret weapon is that the wood contains oils that act as natural preservatives that help the wood resist the fungi that rot wood, and the insects that like to eat wood. These are the same oils that give cedar its distinctive aroma (and why cedar is used in closets and hope chests). Some customers enjoy this cedar aroma and so leave their furniture unfinished seeking to retain the fresh invigorating smell.
Left to itself, cedar will age naturally to a handsome silver-gray patina (with the occasional darker streak - it is a random natural process, after all). The wood grain will be raised a little over time, and be more prominent. Cedar furniture left unfinished works best in climates that don't encourage decay. A longer service life would also be expected if the furniture was kept on decks or patios, rather than lawn areas.
Sealers and Wood Preservatives
Notwithstanding everything said above, although cedar is a naturally durable wood, it must be said that leaving it untreated is not recommended if you want to gain the maximum service life from your outdoor furniture investment.
Your first line of defense against the elements that is the easiest and least expensive (well, other than doing nothing) is to apply a wood sealer (Thompsons Wood Sealer is one well-known brand). These are generally water repellents and water repellent preservatives. The purpose of these products is to reduce the absorption of water by the wood. The pores of the wood are at least partially sealed, and the effects of raised grain are reduced.
In cases where your furniture is exposed to constant ground contact, you might wish to consider a product that is also a wood preservative through the addition of a fungicide that inhibits the growth of mildew and decay fungi. Cedar is naturally insect resistant. However, if you plan to have your furniture in ground contact in termite-prone areas, you should also consider a wood preservation treatment (especially for those furniture parts in direct contact with the ground).
When used as a "natural" finish, wood sealers/preservatives allow the natural aging of the wood. The more sealer and/or preservative that is absorbed by the wood, the better the protection, and the longer the service life. The service life of a water repellent is only one to two years, depending on the wood and the exposure. A good recommendation would be to reapply the sealer/preservative once a year.
The opinion on the use of oils on outdoor furniture is divided. Some desire a little more protection than a sealer or preservative can supply, yet want to retain the natural look and character of the wood. One of the earliest protections for outdoor furniture was oil. Old-timers would soak their outdoor furniture in boiled linseed oil to afford some measure of protection. In our modern times, linseed oil has been replaced by modern oil formulations that perform much better.
An application of a quality teak oil will help protect the wood and give the wood a rich appearance. Follow the instructions of the finish product carefully. Like sealers, the service life of an oil finish is one to two years, and a yearly application is recommended. If you decide to use an oil finish, you will achieve better protection if you choose a product that contains an ultraviolet blocker.
Some feel that an oil finish is unsuitable for outdoor furniture because the oil will capture dust particles in the air. The finish can achieve a "dingy" appearance, and be harder to keep clean.
Cedar is considered to have excellent finish-retention qualities, with an exceptional ability to accept many different types of finishes. It is recommended that if you decide to apply a finish to your outdoor or indoor cedar furniture that you do so before the furniture has weathered much. Even a few weeks of exposure will decrease cedar's ability to hold a finish.
Note: Cedar should NOT be left unfinished for 6 to 12 months as suggested by some finish manufacturers (and others) ].
Tip: Finishing wood that is obviously wet, will likely cause finish failure.
Exterior wood stains fall into two groups - semi-transparent and solid color. Both come in a wide range of colors, and surely one will suit your decor and tastes. Once again, purchase a product with ultraviolet (UV) inhibitors for the best results. Little, or no, preparation is required to stain our furniture. The furniture is delivered completely sanded and stain does not require a primer. Just make sure the surface is clean. Apply a semi-transparent stain (with UV inhibitors) in two coats, following the manufacturer's instructions. To keep your outdoor furniture fresh looking, every other year you might consider giving it a very light sanding, and reapplying your semi-transparent stain. Indoor furniture might go for a decade or more before you felt the need to refinish.
Some (the Western Red Cedar Lumber Association for one) advise against the use of solid color stains for outdoor furniture, maintaining that they are not really suited for smooth, sanded-wood applications. Solid color stains tend to leave a surface film which does not adhere well enough on the sanded wood to stand up to the day-to-day abuse of furniture use.
The service life of a quality semi-transparent exterior stain, with UV inhibitors, is one to three years (and likely only one year without UV inhibitors).
Paint, without a doubt, provides the most protection to your furniture against weathering and wear (especially from the detrimental effects of water). Being an opaque finish, it also conceals your wood's natural characteristics. Paint is available in an unlimited color range, and so is often a choice when your furniture needs to closely match a decor.
As in the advice for stains, your furniture should not be allowed to weather any more than necessary before painting. Your furniture arrives completely sanded. For the best results, a quality exterior alkyd oil-based primer offers the best shield against staining by "bleed-through". A quality exterior latex paint (especially a 100% acrylic formula) remains the most flexible and durable as it ages. The service life for a high quality primer and paint system, properly applied, can be up to ten years. (However, at some point, the furniture will need repainting - requiring a thorough surface preparation, including scraping and sanding. By that time, you'll probably be ready to change the color anyway.)
Transparent, non-flexible, film-forming finishes such as lacquer, shellac, urethane, and varnish are not recommended for exterior use on cedar. Ultraviolet radiation will penetrate the transparent film (even with UV inhibitors) and degrade the wood surface.
No matter how many coats you apply, the finish will eventually become brittle, then crack and blister - and will ultimately fail. If you do decide to apply a polyurethane, or similar, transparent finish - don't say we didn't warn you.
A clear finish using any of a wide variety of clear finishes is acceptable for interior furniture.
As with any interior furniture, exercise caution in allowing the furniture to receive too much direct sunlight.
The naturally occurring oils in cedar give it a certain amount of resistance to termites. Generally if other food sources are present, a termite (or other wood-eating insect) would just as soon not dine on cedar. However, we recommend that if your cedar product is going to sustain constant ground contact (especially if it is dug into the ground, as perhaps our Garden Arbor or Cedar Trellis might be), that you treat it with a quality wood preservative. This will help ensure many years of enjoyable service.
So, the finish options are many for your cedar outdoor and indoor furniture, ranging from doing nothing, to various finish treatments to suit your needs and style. Finished properly, with high quality materials, your cedar furniture will give you decades of trouble-free service. If exposed for prolonged periods to conditions where decay might be a problem (such as constant, direct contact with the ground), you should treat your furniture with a wood preservative.