I had a beautifully varnished sitka spruce mast on my Gulf 40. It was varinshed (by the PO) all the way to the very top - no paint. One day while out sailing I went up to the mast to tighten the jib which was sagging slightly. As I looked at the jib and began to crank, the winch pulled partway out of the mast in my hand (yes, that was a sickening feeling...). Water had over the years wicked its way up from the bottom at the deck step plate, and about three feet up the mast the wood got soft from the inside out until the three-inch long screws holding the winch in place pulled out.
Anyway... Long story short, had the mast pulled and fixed at Schooner Creek Boatworks in Portland, OR. While it was down, more soft spots were found at the very top of the mast where a lot of the harware was attached. It was because the varnish started peeling and blistering at the wood/metal points, way before any other part of the mast began to show any signs of UV degradation. So water got in at the top. Just like Plumper said, paint up there is a lot easier to work with and won't fail as quickly.
The yard owner, Steve Rander (who is an awesome boat guy), told me the "rule of thumb" for painting wood spars was to paint the top 1/5 to 1/6 of the mast.
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"Always approach the dock at the speed you wish to hit it."
1978 North Sea 33 Pilothouse Cutter (Ta Chiao)