First, what type of wood are you using? Different woods soak in moisture at different rates. Some will need a very heavy, very protective finish to keep them in good shape despite exposure to son. Some will need only a thin coat because the wood has such a tight grain and structure that water can't soak in easily. Because of that, not only will you need different volumes of finish for different woods, but some may soak in more easily to some kinds of wood. Additionally, some types of wood contain oil or sap that could repel the finish so you need the right one for the job.

Second, what kind of conditions will your deck be withstanding? Is it standing in the sun all day? A deck that is in direct, hot sun all day can lose moisture fast. Sure, wood is supposed to be dry and is dry when you use it to build a deck, well, mostly dry that is. Wood needs to retain some moisture, a small amount, or it ends up weathered and splintery which isn't a happy thing for wood. If your deck is subject to lots of hot direct heat, make sure your finish is formulated to withstand that. Is your deck in a moist climate? If so, what kind of moisture? Can there be more than one? Actually, yes, there can be more than one kind of moisture.

First you have cold and continuous moisture. That would be somewhere like Seattle or the Oregon coast. This kind of moisture is continuous and can lead to some lovely mold or lichen growth. Lichen can be very destructive to wood. If you live in a cold moisture area, you may get a wood finish that also helps repel growth on the wood.

Another moisture is hot and humid moisture. This is more likely to "pop the grain" which is what happens when wood is too wet for two long. With this moisture, the sun helps cause the damage much faster. You want a finish designed to repel the sun and water both.

Also, there is salty moisture. Areas by the ocean have a lot of salt moisture which can be a bit of a contradiction because by nature salt is a substance that makes things dry, and it's coming in moisture which of course makes things wet. However, you can find a treatment for that as well.

The last factor to consider is friction. This can be based on the volume of people using the deck, the furniture or children's toys that will be dragged over it, or also on wind. When you see old, grey, weathered, wood, it is typically caused by moisture and wind. Wind can be very harsh on wood as it will suck the moisture out, then when any moisture hits the wood it's sucked in as replacement, and then the wind sucks it out again. If you're in an area that gets a lot of wind, make sure that you have a finish designed to really retain the moisture in the wood, to repel any outside friction, and then put on the maximum number of recommended coats and refinish as often as is recommended by the manufacturer.