Most all decking products are dirty scarred, and marked up as they come from the local lumber yard.
If you put a finish on top of these marks, you highlight them and they are more easily seen.
You can have a floor sanding person come in and sand your deck, but it will take quite a bit more work to hand sand all the railing components.
JrMillworks sands, or planes or cuts all surfaces of all post caps and railing components in an effort to remove all dirt and grime and marks so you are ready for your finish, and your railings look like furniture.
Here's a bunch of 2x2's direct from the lumber yard. This is typical of what you get. I put the worst side of each board up for view.
The manufacturer waxes the ends of the board. Every batch has wax that needs to be removed.
More wax. This much wax requires at least 2 passes through the sander.
Here's one from above. This piece will have to go through the sander 2 times on this face to get to the bottom of this grime.
Here's the board prior to first pass in the sander.
The first pass through the sander took most of it off, but remnants of grime still remain. This piece will need another run through.
The 2nd pass finally got it clean.
Can you see the "skip dressing" or planer marks on the board on the right? These marks are too deep for the sander, I'll have to plane these marks off.
More "skip dressing" on the 2nd board from the left. Out of 30 boards, there were 6 that needed to be planed because the damage was too deep for the sander. Believe it or not, this was a decent batch of lumber. I haven't seen a load yet that didn't really need a lot of work to clean it up.
I've found that it's better to sand all 4 sides even if you don't think they are stained or soiled. When you look at the board from another angle or another light often you see blemishes that you didnt see before, so I make it a point to sand all 4 sides of the 2x2 ballusters regardless of what I see.