could be more unknown electrical leaks - esp if blisters are round.
Could also be 'salts' - we don't have products for it, but do have an
info page with links on it explaining it.www.epoxyproducts.com/salt.html

paul oman
progressive epoxy polymers

Hmmm.... yes, the blisters are definitely round. And it happens only
at and below the waterline. Probably you are right and there are more
electrical leaks. Actually, I also found that there is a potential on
the hull, so something else must be leaking. It is a very frustrating
problem. It takes so much time to repair all those blisters and if I'm
unlucky I've to replace the propellor again too

Thanks anyway,
Joost.