As a wrap to this thread, a few days later I was leafing through my latest copy of 'Good Old Boat' magazine and came across this sidebar to an article. It matches the most commonly used sealants against the most common boat materials. Source is attributed to Don Casey, author of the 'This Old Boat' series of books, a pretty reliable authority. But note in the fine print it's lifted from his latest edition in 2009, so may not be up-to-the-minute timely.
Posted for info (hoping the attachment attaches).
Joe