Be aware that the foam can begin hardening at the air exposed surface while further inward it is still expanding, thus forming a dam and pushing things out of shape. That's what happened to my garage door. I filled it partially near the bottom 18" (30" wide hollow core door) and expected it to expand upward. I may have filled a bit much for the compartment, but I had 3/8" vent holes every 8" or so. It wasn't enough.

I might do this on a kayak I'm outfitting for my Mother Inlaw: shape a bulkhead from styrofoam and then use expanding foam like caulk around the edges.

> jim i have not used the sealant you are talking about but i have
> used GREAT STUFF, for many projects it is a canned foam you can get
> at Home Depot for a few dollars a can. It is a waterproof closed cell
> foam, it will degrade in sun light but that won't be a problem for
> your application. This foam expands ALOT!!!!! (do not believe the
> label when it says minimal expansion) Not only does it expand alot
> but as it expands it hardens and if it has nowhere to go, (if it is
> in an inclosed space) it will deform that structure ( i have seen
> the foam used to seal a house door that was working fine , the next
> day when the foam was cure it had twisted the door jam so much the
> door would not operate)

> The best way for you to use the foam would be to install it before
> you install the deck that way the foam can expand up then you can
> cut off the excess when its cured.

> The only other thing is that no matter what foam you use it is
> expensive so a trick you can use to reduce the amount of foam is to
> place a few empty capped 2 liter soda bottles in the bow and stern
> then use the great stuff to fill in the space around the bottles.
> this way you could add flotation to your kayak for the price of 1or2
> cans of great stuff.

> I plan on using this trick on my kayak a skin on frame.

> don't worry about the bouyancy of the soda bottles they actualy
> have more bouyancy then the foam for equal volumes.