I've heard of people using paint rollerer sleeves. You just put your stain or sealer in a 10 inch painting tray, screw your paint roller frame onto a threaded pole, and proceed just like you were painting a floor.

I've spent enough time on these DIY Q&A forums to find that people have lots of good things to say about Cabot deck stains and sealers.

Also, to be clear, the purpose of a sealer is to prevent fluid from penetrating into or evaporating out of the wood. Wood swells when it gets wet and shrinks when it dries out. The problem is that wood doesn't absorb moisture equally on every surface of the lumber. Wood absorbs and evaporates 15 times as much moisture 15 times as rapidly through the end grain of the wood. So, applying the sealer to the top surface of the boards is good, but where it's most needed is at the wood end grain. If it were me, I'd stain and seal the top surface of your deck boards, and then spend a day just painting the sealer onto every exposed end grain you can find on that deck.

Also, I'm no expert on this, but it seems to me that staining and sealing the sides and undersides of the decking boards wouldn't be necessary except at the end grains. I've never seen wooden windows rot anywhere where water could not accumulate. So, caulking those places where you can't seal would be a good idea too.

And, so you know what to look for, UV deterioration of wood results in the surface of the wood turning grey and fuzzy. Cellulose absorbs ultraviolet rays, and so the depth of penetration of the damage is generally very small, and you can clean up the wood quickly and easily by just sanding or brushing that grey fuzzy wood off.

Moisture damage causes wood to split, and if it's not securely nailed down, to cup and twist as well. The splitting is the direct result of wood absorbing more moisture faster at it's end grain. What happens is that in a rain, the ends of the boards (if unsealed) will absorb water, causing the wood to swell at the end grain. Water is then carried by capillary action from the end grain of the wood longitudinally along the board so that wood cell walls a few inches from the ends of the board become saturated with water and swell. Up until now, everything is fine and the board isn't damaged. Now, the rain clowds part and the Sun comes out and the end grain of the boards evaporate moisture 15 times as fast as the wood a few inches from the end grain. So, the end grain wood shrinks while the wood a few inches from that end grain remains swollen. That puts the wood at the end grain in tension, and it splits if it cannot accomodate or withstand that tension. So, splitting of the wood at the ends of the boards is an indication that you need to seal that end grain better.