Aside: Why does the dust, even from week-old completely dried paint, clump up on the sandpaper? And why do the clumps keep reforming in exactly the same places, after you flick them off?
After sanding, I wiped it down with rags and paint thinner to get the dust off, and we brought it back inside after the thinner evaporated.
Finally, finally, I applied the first coat of Interlux single-part urethane enamel, using the roll and tip method to the first horizontal surface. I can't say enough how much I like this paint - it goes on like cream, and levels itself to a mirror-like finish. The aggressive self-leveling kept me from rolling the bowsprit over to get the next side. When I tried rolling it over too soon, it began to sag. Roll it back! Roll it back!
It looks like I will be able to get three sides coated in a day, but I will have to do them one at a time. Yesterday, I only got two, but I spent a heavy morning sanding before I started. Today, I got the other two. Next weekend, I should be able to do as many as three sides in a day, tho now that I have a coat on all four sides, the system is thrown off.
I won't be able to avoid inter-coat sanding - it'll be necessary to remove dust nits, etc. But this should be just a light scuffing with 150 (the paint fills 150 grit scratches completely).
Did I say I love this paint? The bowsprit looks and feels like it is coated in plastic.