Nothing. You don’t need to do a thing if you are OK with letting it gray over time. It’s pressure treated.

Put a roof over it and walls around it with lots of windows.

Define "best".

" DH would like to use Thompson Water "

Thompsons Water Seal is absolutely garbage and if you ever want to use anything else will be a real bear to go over inside of 5 or more years.. Typically if you use PT wood it is not treated with a clear preservative, usually if anything it is stained after a year or so. An Olympic Clear Wood Preservative would be my choice.

@millworkman Totally agree re Thompson’s. Will look at Olympic. I understand that sealing pt deck, is necessary to prevent, mold, mildew etc.

" I understand that sealing pt deck, is necessary to prevent, mold, mildew etc. "

Sealing may help but it will not prevent mold or mildew. if it doesn't get sun to dry it out that will happen regardless. It sill help with some color fading and checking but only so much and for so long. PT wood is typically yellow pine and yellow pine does what yellow pine does.

@millworkman The deck is north facing. However, I will heed the wisdom of the houzz, hive and forego any treatment. This sure will save time and $$, energy too.

Our north facing PT deck is covered and nearly 5 years old. Recently had it pressure washed and stained with Pro Luxe.

@fnmroberts Your deck looks great.

Our deck has composite decking, but pressure treated railings. After a year, I used ReadySeal, in pecan color. It's raining right now, a year later, and you can see how the rain beads up nicely on the rail. (yes, I feed both birds and squirrels).

@AnnKH Thank you for the info. Love that you feed both, I assume, squirrels cant raid birdies’ food,

We are also in Canada, but in Vancouver, so more rain than snow. Our deck isn’t treated. It’s not pressure treated though. It’s cedar and covered. We’ve let the stairs and our fence weather to grey over six years.