In this instructable, we will talk about how to protect an outdoor wooden piece of furniture from the elements. This is only about the finish to apply to the wood, and not about how to make a specific piece of furniture. The examples used to illustrate this instructable are an adirondack chair (described in this instructable) and a patio table of my own design.

The problem behind the need to protect the wood is that an unprotected piece of furniture will not hold a season outside, for the following reasons:

The wood absorbs water when the air is humid, and lets water out when the air is dry. This makes the wood shrink and swell every now and then, and can create cracks in the wood, or break joints if they are too weak. However, with a proper design, wood movement can be accounted for, to let the wood shrink and swell without forcing on the joints.
With too much humidity or stagnant water, the wood will start to rot, and fungi will start decomposing the wood until it has no structural properties, and falls into pieces.
The UV in the sunlight will fade the wood colors until the wood is completely gray.

With that being said, the best way to protect a piece of furniture from the elements is to paint it with an outdoor paint. This is the cheapest, most effective method, and it requires the least maintenance. If you follow this route, make sure that you completely cover the piece (apply paint behind and below the piece too). Otherwise, the tiniest uncovered surface will allow moisture to get in.

However, once a nice piece of furniture has been made with expensive wood, one often prefers to let the wood be shown, and not to cover it with paint. There are several ways to protect a piece of furniture while still leaving the wood visible:

Exterior oil
Exterior, spar, or marine varnish
Epoxy sealer with an exterior varnish topcoat

If you prefer a protection that is easy to apply and cheap, but requires frequent maintenance, choose option 1 or 2. If you prefer a protection that requires less maintenance, but is more expensive and labor intensive to apply, choose option 3. This instructable will give guidance on option 3 "Epoxy sealer with an exterior varnish topcoat".

Supplies

Epoxy resin

Exterior varnish (spar urethane)

Sandpaper

Gloves

Stainless steel screws

Step 1: Dry Assembly

Once you completed and sanded your piece of furniture, but before you apply anything to the wood, assemble the pieces. This way, you can still make adjustments for the pieces to fit. After applying the finish, it will be too late to make adjustments. It is important to drill all the holes for the screws, and make all countersink holes for the screw heads.

Use screws rated for the outdoor, such as stainless steel screws (304 stainless steel is good enough in most cases, 316 stainless steel is needed if you live close to the sea).

Pre-drill before inserting screws, and make sure that the drill size is appropriate for the screws that you use. You can find the correspondence on this website: bolt depot instructions

Be careful of not locating the screws too close to the edge of a piece of wood, or this might become a weak point and break first.

Once the furniture is completely mounted, disassemble it to apply the finish. This way, the finish is applied all over the pieces. If the finish was applied on the assembled furniture, there would be small interstices where the brush could not go, and therefore would not be covered.

Step 2: Epoxy

After the piece of furniture is disassembled, apply the epoxy. The epoxy is used to prevent moisture from getting into or leaving the wood. This way, each piece will keep the same dimensions, even when the air humidity changes outside.

There are all kinds of epoxy on the market. Be careful of the color (you might want a clear epoxy), the curing time (5 minutes is too short, 15 minutes is perfect if you can apply it quickly) and the thickness (look for an epoxy without thickener, so that it penetrates deep into the wood). I use this one: amazon and it works quite well.

I apply the epoxy in 3 steps:

I first fill all the holes, dents, and cracks with epoxy, let it cure and sand
I then apply a first coat to all of my pieces, making sure that I completely cover everything, if possible even the holes for the screws (they might have to be redrilled at the end if the screws don't fit anymore).
After the first coat has cured a little bit, but is not hard yet, apply a second coat. For the epoxy I chose, this was after approximately 4 hours.

Make sure that the 2 components of the epoxy are mixed well before application. Otherwise it will never cure.

Each coat should not contain too much epoxy. Otherwise, drops will form on the side beneath the pieces, and it will be harder to sand. If you waited too long to apply the second coat, or if you need to apply a third coat, wait for the last coat to cure and harden completely, sand, and apply the new coat.

Coats can be applied with a paint brush (pick a cheap one, as you will likely not be able to reuse it), or I found that applying it by hand works well too: wear a pair of single use gloves, take some epoxy in your hands and spread it on the wood.

Once the last coat has been applied, let it dry for at least 2 days.

Sand with a 120 grit sandpaper. Be careful not to leave swirl marks behind during sanding, as these will be visible on the end product. Here's a good video on how to avoid them: youtube

Clean the pieces with a wet towel and let them dry.

Step 3: Exterior Varnish

The exterior varnish is used to protect against UV light. Without the exterior varnish, UV would deteriorate the epoxy. Make sure that the varnish is rated for the exterior. It usually says spar urethane, marine varnish, or exterior varnish. Here's the one I use, and it works quite well: Lowe's.

Varnish should be applied with a natural bristle brush. Don't apply too much varnish per coat. Each coat should be thin to properly dry. The first coat should be left a little bit longer to dry (approximately 2 days), while the next ones dry in about 1 day.

Sand with approximately 220 grit sandpaper between each coat, then wipe with a wet towel, and let it dry.

Apply at minimum 3 coats.

Be aware that varnish will give a red-orange tint to your wood (see picture).

Step 4: Final Assembly

After the finish has been applied and has dried, you might have to quickly redrill the holes for your screws if they don't fit.

When assembling, you should do something to prevent moisture from getting through the screw holes. If you don't plan on disassembling your piece later, you can dip your screws in epoxy. If you plan on disassembling your piece later, you can dip your screws in petroleum jelly, or wax.

Even though a great protection has been applied, you should periodically check your furniture for any damage, and reapply varnish when necessary. This should make your outdoor furniture last many years. Let me know in the comments if you have any other trick that you use to protect your outdoor furniture, or if you have any question.