Let's start at the beginning.
Have you built a SOF out of PVC pipe?
Why do you want to stiffen it?
How big is it?
How much does it weigh?

: im a little new to the kayak building world and could use some help
: with a question about a prototype folding kayak that iv been
: building.

: ok so first off let me describe my designe: I have taken a basic
: yost style folding kayak frame designe. except i have replaced
: the aluminum stringers with stringers made of interlocking
: sealed sections. so fare this has worked exceptionly well on
: lakes and with floats is also exceptional on the local harbor.

: now as for my question i have been fooling around with the idea of
: filling the center of the pvc piping with some sort of folm. i
: bealive this wold add strength to the frame. but i am not sure
: wether this wold aslo add boiancy th the kayak or if it wold
: reuin the boiancy created by the air
: traped in the sealed pipeing ?

: thank you for any help and i will post pics as asap