You don't say whatkind of wood the deck is made from. Here, in the U.S., many decks are made from mountain cedar since redwood has gotten prohibitevily expensive. Cedar is a good, weather resistant wood, but has a couple of drawbacks. One, it does turn gray unless cleaned and treated every two yearr so. Secodly, it has an oil within it that lends to the the weatherproof qualities, but if you power wash it, using a wire brush or sander, you'll want to wait for a couple of weeks for the oil to leach out of the newly exposed surface.
If it is a cedar type wood, you can't go wrong with a good quality semi-transparent deck sealer. The semi-transparent has just enough coloration in it to brighten up the deck, but not so much that it looks like paint. Here, Behr brand is a good quality.
You might also look for Thompson's water sealer for decks.
BTW... these finishes will work even if the decking is not cedar...