Thanks for the suggestions. I don't have a cabinet scraper but last night I gingerly tried scraping a couple of areas using a single-edged razor in a different kind of scraper handle. I stopped after the sharp corner of the blade caught on some wood and lifted up a small shard.

Next I "finger sanded" some areas (80 grit) and found I could at least lighten if not actually eliminate the stains. Following up with block sander and 120 grit smoothed the area nicely. Some stains went away entirely. Judging by the cross-sections at the hatch holes, there is enough meat in the 1st ply to allow working in this fashion. It's extremely slow work but it's better than leaving as is, even if it's not perfect.

As an alternative, I suppose I could just paint the whole boat. :-(

I was looking at an old issue of Wave Length, in which two women found that the paint they wanted to use was incompatible with epoxy. They paid an auto body/paint shop to finish it in polyurethane paint. Not my preference, but...

BTW, in the same issue there is an essay by Rod Tait. Is that you? It's called
"The Moaning Chair." I too have a moaning chair. And a moaning sofa. Heck, it's a moaning workshop.