When you ask "If the barrier coat between the keel and hull is in
good condition", and also note Joel's advice on refinishing the cast
iron keel, it sounds like you're removing the keel,(the only way to
see if there's barrier coat between the keel and hull).
Unless you've got some bad damage, this seems a pretty drastic and
unnecessary maintenance item. Also, I doubt that Amel would have used
a barrier coat on the underside of the fiberglass keel stub/water
OTOH, perhaps you are making a distinction between two sections of
the fiberglass hull, that is, between a.)the fiberglass hull
structure from the water line down to where it turns straight down on
the side of the keel stub/tank sump and b.)the fiberglass hull
structure from that turning point at the top of the keel stub down to
the cast iron shoe plate.
If so, it sound like you have observed barrier coat on b.) but not on
a.) - which seems a bit odd.
Be that as it may, if you're going to strip the entire fiberglass
hull (from waterline to the top of the cast iron keel, you'll
certainly be removing the barrier coat (if there is any on your
With the fantastic history of Amels not blistering, you might simply
skip the barrier coat altogether. Of course, you'll want to
chemically stip, rather than sanding so you don't affect the
integrity of the gel coat. (If you did, adding a barrier case would
seem prudent, indeed.)
I stripped my boat down to bare fiberglass 8 years ago (same "caking"
of old paint you described), and did apply barrier coat plus bottom
paint with super results. Of course, whether it's the super quality
of Amel's fiberglass (I've read that'ol Henri was nicknamed
the "father of fiberglass" in France) - or the barrier coat that has
kept the blisters away is anybody's guess. I'd bet on the former,
but wearing both belts and suspenders is fine.
As to primer, just follow your paint manufacturer's instructions - as
Joel says, "Read the instructions and read 'em again."
Good luck with your project.
Craig, s/v Sangaris - Santorin #62