Outdoor wooden furniture takes quite a lot of abuse, so to keep the elements at bay you need to protect it with a good sealant and spar varnish. Read on for more information about getting the job done right.
Prepare the Wood
The wood needs to be well-sanded and cleaned prior to any application of a clear sealer. Start with a low grit paper, such as 80, and build your way up to 220. This does take some time, but when applying clear finishes such as spar varnish, the attention to preparation is the most important.
When the wood is smooth to the touch, it is time to clean it off chemically. When you are using a urethane-based product, such as spar varnish, you will want to gently wipe off the wood surface with a clean rag soaked in a bit of lacquer thinner. Let this air dry for 15 minutes or so, and then take a tack rag and clean off any of the foreign matter that may have settled on the surfaces. Tack rags are bits of cheesecloth impregnated with gum arabic, typically, and will pick up almost every bit of dust or dander that might spoil the finish.
Apply the Spar Varnish
Use a clean bristle brush—not nylon—to lightly coat the furniture with the spar varnish. When applying varnish coats, it is best to use a number of thin coats, rather than a single thick one. Let the varnish dry completely before doing a light, dry sanding with more 220-grit paper. This is to cut down the inevitable bits of fluff and dust that may have settled on your piece during the drying time.
Wipe this off with a damp rag, and don't be dismayed if it looks "white." The clear particles of the varnish have been rubbed raw by the sandpaper, but this is what we want. Apply a second coat of the varnish, again, keeping it quite thin. This will be the time where the wood will begin to gain some of the luster that we are after. But we aren't done yet!
Using steel wool or a polishing pad, go over the surface again, just like you did with the 220-grit sandpaper, after the second coat has dried. Once again, take a damp rag and wipe down the surface, removing any of the dust.
With the third coat, apply enough varnish to make the surface look wet, as if it had been dunked into water. Avoid drips and puddles by not laying it on too thick all at once. This coat will take the longest to dry; leaving it overnight is best.
Now, lightly burnish the wood with steel wool or your polishing pad. Be sure not to scrub the surface; just brush it well. The damp rag will remove any of the fluff, and then you can apply the final coat of spar varnish.