J wrote:
The issue may be in the details, ie what is an "enclosed space, no
air, etc. I think those foams rely on moisture in the air to cure,
if I'm not mistaken. So, if you really put it in something that is
totally airtight, I can see that being a problem. But I bet the
area around a spa tub doesn't meet their meaning of airtight. I
think enough air is going to get to it so that it will cure.

Precisely. I'm sure that for any "surface" area (within an inch or
so) it should be fine. It's just that I think there will still be
plenty of tunnel spaces (surrounded by the previous foam, which
should be air-tight) that will probably not get the necessary air for
good curing. But upon reflection and reading the responses, I think
the Great Stuff "big gap filler" will be good enough, even if not
perfect.

Thanks all.

P.S. I think the 2 part expanding foam mentioned by Bob F. would
probably be too difficult to get into place for this scenario -
though the foam itself would probably work better.

A syringe and connected plastic tube (straw?) should be able to do it fine, and
the 2 part foams are not going to have curing problems.

Any foam might cause problems with too much expansion if too much is used.