So all you home builders (with solid masts) how did you do it?
Gareth,
with an electric planer, a prism mast support, and some discipline when turning the mast on systematically. You start of with a square section. Then plane down the corners until you have an even 8 corner section, then 16.
I stopped there and did the rest with an electric sander. Do that in the evenings when the sun is low so you can see the remaining ridges (corners facing up) better.
From close up such made masts still look a bit rough, but from a few yards away they look perfect. Good enough for me, and I heard no complaints ever.
I did 3 masts this way (2 for my Drascombes, and one for my SeaRaider as spare), and they got a bit smoother with every new go. So it's not just elbow grease, it's practice too.
But then, solid masts are dumb. They reduce the stability through their unnecessary weight. There are loads of tips on the www how to make hollow masts, it's not rocket science.
Good luck.