Just remember, even after painting all (which I do), there is still a chance that you will see the rifling marks caused by the drill - especially if the kit calls for a thin wall.
Unless, I am making a custom pen (kitless), I only select blanks that will hide the rifling marks caused by the drill. You can drill and mandrel to sanding the inside of the barrel. However, that may change the inside diameter of the barrel which will cause the hardware to fit loosely.
If I am unsure about the transparency of a blank, I first drill the blank (or use a cut-off from the blank) with the recommended drill, then without a tube or bushings inserted, I turn the blank about 80% down to final dimensions and profile (narrow ends where the h/w meets the blank are often vulnerable to seeing what is inside the barrel.)
I then slide he bare brass tube inside the barrel - make sure there's plenty of light on what you are looking at; you will quickly see if painting with help hide the tube. Remember, not to over tighten the tubeless blank between your centers and that your tools are verb sharp. You don't want to crack the blank or chip the end-holes. This saved meh much aggravation, and now I have a good list of non-wood blanks that I enjoy turning without seeing the inside of the barrel/tube.
You will find what works for you.