Good advice from Mr. Bill, rokket and Siara.
There are two things happening when you score/sand/file a surface before using glue or paint.
The first is that you microscopically are creating more surface area for the solution to adhere to, in some cases you may even be doubling the area, which of course makes for a better bond.
The second is that some of the scratches actually undercut the surface, which lets the hardened paint/glue key into it, much like a dovetail joint in carpentry.
As for staining the deck as rocket mentioned, that