pour in place foam

[jay-jay]

I am restoring a 1973 Starcraft Chieftan, and want to put urethane pour in place foam for flotation under the floor . Is it best to pour in, then lay the floor.
Also should I leave the center section empty between the floor supports or can I fill the entire under floor area? ,to stiffen the entire hull.

[Noah's]

If you pour the foam with the floor up you will have a lot more control over coverage. Mix about a litre, pour within 2 minutes, it will expand fully within about 20 minutes. Add as needed, if it gets to high in spots, just trim with a bread knife. Filling the entire cavity will add some temporary stiffness but this is not a structural foam so will break easily under stress.