And it is clear to me that with end grain pointing skyward, unless the top is meticulously maintained, water will find it's way into the end grain. My spar, wooded here - by me 10 years ago. The mast head is a vulnerable place to allow rain intrusion. But I get a close look at it each year and it spends winters in a shed(these days). The tolerances are so tight that spar varnish does the sealing of all the tiny bedded joints between metal and wood, and wood to wood glued joints. I lay it on those joints between tangs, sheeve box, through bolts. Varnish is a good moisture sealer if it's applied annually.

The 'button' you speak of may well have saved my friends schooner main mast. Once the rot starts, water has an increasing dimple around chain plates and tangs, to fill and soak into the grain as the mast was in the rain year round. It just needed a tiny roof over the mast head, the sides repel water automatically.