Things You Will Need
Redwood cleaner and restorer, Sanding sealer, Pigmented stain, clear finish or paint, paint brushes, soft cloth
Select the type of finish you want to apply to the redwood trellis. Clear water repellent finishes even with UV protection added usually will not prevent the natural color changes that happen as redwood ages. They also will require reapplication every year. Pigmented stains and sealers last longer but can darken the wood so I recommend using a sanding sealer first before applying the finish. Paint is probably the most durable but in my opinion why use a wood as beautiful as redwood if you are going to paint it. Pressure treated wood would work just as well and be less expensive. I usually finish redwood with a quality pigmented stain and sealer combination after a coat of sanding sealer.
If the redwood trellis is not new, clean it with a commercially formulated redwood deck cleaner and restorer following the products directions. The cleaning products can be purchased at most any home center, hardware store or paint store. Spray the cleaner on and scrub with a soft bristle brush. Make sure you rinse thoroughly to ensure the cleaner is removed completely. Let the trellis dry for a couple of days before beginning the finishing process.
If the trellis has not been assembled, I recommend the finish be applied before assembly and a second application after it is assembled.
Apply the sanding sealer with a brush and let dry completely following the manufacturers directions. A very light sanding is recommended (but not always necessary) to increase adhesion of the finish coats.
Test the finish on a small area to be sure the color and finish is what you expected.
Apply the pigmented stain and sealer again following the manufacturer's instructions. Some stains provide better results when brushed on while others are better wiped on with a soft cloth.
After the trellis dries, (usually a day or two), place it in its' desired location, add your plants and enjoy.
Redwood takes and holds finishes better than most woods and with proper care can look great for years.
Tips & Warnings
Linseed oil is not recommended because it is slow drying and provides a food source for mold and mildew.
Whenever working with tools and or chemicals, safety glasses are recommended. Masks are also recommended to be worn when working with some types of wood and wood finishes or paints to protect your lungs.