Both my Cavalier 32 (10m NZ sloop) mast
and boom are bare alloy. The boom was painted and the paint in bad condition when I bought the boat
3 years ago. I had it sandblasted and then I immediately painted it with a 2-part clear penetrating (thin) epoxy
; just one coat as a sealer. I have not had any issue with staining on the mainsail
cloth, but I always put the sail gaskets between the sail and the boom, not around the boom, so the sail isn’t squashed onto the boom.
on proper marine
alloy is not a structural problem; just think about all those unpainted alloy French hulls.