The exterior wood on the top of my garden office / summer house is looking a bit tired / worn out - see picture below. What kind of treatment should I use to restore/protect the wood? See the lower parts for how it should look.
Home Improvement Stack Exchange is a question and answer site for contractors and serious DIYers. It only takes a minute to sign up.Sign up to join this community
Generally you want to treat exterior wood like this with a water-proofing treatment at least once a year. It helps to provide UV resistance and keep water out.
Water is your big enemy here. It looks like the wood was installed with some lacquer for waterproofing (hard to tell from the photo), but it's worn away. You've got a couple of options
Paint will waterproof and make it look nice and uniform again. It sounds like you want the wood look, but as far as options go, this one is the easiest and had the fewest questions on maintenance. I mention it for completeness.
I'm assuming you just want to work on the greyed spots. This is trickier because matching stain is difficult. Just like painting, your best bet is to stain the whole thing. The next best option is to find a stain that's close enough. Because you have a natural break in the soffit and wall, a close enough color might do the trick, and people will assume the wall just looks different due to lighting.
Then you'll want to waterproof it. This isn't a "one-and-done" either. Your wood will grey over time. The more often you waterproof it, the longer it will stay the same color. Waterproofing also helps prevent the boards from warping and splitting over time (water is your enemy there).
Oxalic acid is a wood bleach that I have used many times to restore wood back to its original color. It comes dry in a container and is to be mixed with hot water if possible, and used in warm weather for best results. It can be done in cooler weather and with warm or cool water, but the results will no where near the same.
Read the instructions carefully, use PPE, and rinse rinse, rinse. A long handled scrub brush will come in handy for all steps. If it is not rinsed off completely, it will leave a white haze, and you won't see it until everything is dry.
Let it dry for a few days and finish with an exterior grade clear finish. This will keep the look of the wood you want for while. The real trick s to get the finish into the end grain of the siding, where it will soak up the finish, rather than soaking up water when it rains next time.