A friend is restoring an old fiberglass Moth sailboat. The side decks/
floatation tanks are very thin and flexible, and he wants to give them more
support by filling the tanks with expanding foam.
He has both 4 lb/cu ft and 8 lb/ cu ft foam liquids, and plans on standing
the boat on the stern and pouring the mixture in several stages, so the
expansion doesn't get out of hand.

I've not used this stuff before, so can't help. Is there anything he should
be aware of before he starts? How large a hole should he have for expansion
of the last batch ? Can he mix component "A" of the two densities to yield 6
lb/cu ft? Any other advice?