I plan to build a roof for my front porch that will connect to the
steep-pitched roof of my brick cape cod house. I want to use 2 square
columns, about 7" or so wide by about 8' tall.

These columns will support beams coming from the house. The beams
support the rafters of the porch roof. Since the columns will be
load-bearing, I want them to be solid instead of hollow. I was
thinking about pressure treated wood.

But I want the columns to be painted white. I don't have experience
painting pressure treated wood. I'm thinking it might now work well.
Any ideas? Should I cover a center post with better wood for painting?

I would buy the prefab kind of columns with the metal tube down the
center but they don't have any plain smooth square columns at Home
Depot. I don't want something fancy. Any ideas?