We had our deck built of pressure-treated pine in 2009, and left it completely untreated up until a couple weeks ago. This is in south-eastern Minnesota.
So far, I've power-washed (and a week later, sanded with 80grit paper) railings, deck boards, steps, and pretty much every area that may come in purposeful contact with skin.
In the interests of time and effectiveness (aka, being lazy,) I did not wash the outer surfaces of the deck, specifically the railings and balusters (spindles). Only the surfaces that you would see when sitting / standing on the actual deck are brand-spanking clean. The unwashed areas are certainly old and weathered gray in color.
We picked up a few gallons of Cabot semi-solid oil-based stain and other sundry supplies today, and are planning on staining the entire deck tomorrow.
Here's my question: is staining the still-gray surfaces acceptable? I'm not worried as much about how great it will look as how long the stain will last and how decent it appears. I'm confident that the prepared surfaces will look pretty much perfect, and am probably worried that there's going to be too much of a differential between both types.
UPDATE, almost 2months later: We finally finished staining the monster. Getting stain in some of the hidden areas was a bit of a challenge, but in the interests of doing a good job, stuff had to be done. As before, the outer surfaces of spindles haven't been powerwashed, and I hope to post an update a couple of years from now, as far as status and longevity goes. Thanks, all!
Best Answer
You can likely answer your own question with a little experiment. Before committing the whole deck to large amounts of stain application try a small area on 1 or two boards where you've sanded on one side and the weathered grey is still on the other side. If you like the result you will know what to do. If you dislike the look or match then you can proceed to close up the stain can and go back to sanding and scrapping the rest of the deck surfaces. The place where you put the stain sample over the weathered grey can be sanded down to fresh wood right along with the rest of it.