sal wrote:
Hello boat people. I am trying to rebuild an old cedar lapstrake boat .
advice please. Some of the hull planks 3/8" cedar have longitudinal cracks
and I would like to fill them with some type of compound not sure what to
use. I understand lapstrake design are hard to rebuild as covering with
cloth and epoxy are not recommended.

Ron-
I agree with your idea that covering with epoxy (and cloth) would be a
bad idea for your cedar-strip boat.
I understand your message to mean that the planks have
split/cracked-not that the joints between the planks have opened up.
Although I haven't repaired this problem in a boat, it's common in the
guitar-building and repair hobby- another interest of mine.
Usually there is dirt in the crack, which makes repair tricky. First
thing is to get the humidity level up to near what the boat will
experience once it is back in use- if the boat's been stored in a hot
barn or similar, it will have dried out and that will have caused the
cracking. If you fill the crack when the plank is very dry, it may
cause problems later when the wood expands.

The general approach I've used is to run a (thin) saw blade in the
crack to clean it out and glue a thin piece of wood into the crack. I'd
use polyurethane or epoxy glue for this, in a boat.

Hopefully some other folks here will have more first-hand knowledge and
the benefit of experience.

Good luck with your repair project!

John