Why not use "Great Stuff" instead of the classsic 2 parts expanding foam??

Thread starter #1

Hi guys as many before im in the process of reducing weight of my sunfish (its on a fan diet :p), the foam blocks got a little detached from the hull, NOW the classic advice is getting a 2 parts foam mix some of it and voila!!! (same process as original manufacturer)
So my question is why not simply use this expanding foam instead of measuring,mixing, applying, etc the traditional foam
One of it uses is in plumbing so I don't think it absorbs water

any input is welcomed thxs in advance

Once the surface skin breaks it is a sponge...

Its use in plumbing isn't a valid test for usefulness as flotation foam in a boat. flotation foam is a safety item. This stuff is insulation to reduce heat loss and block air flow.
Thread starter #3
so any "over the counter" solution?? (easy to buy local hardware store). The 2 parts epoxy is not particularly expensive but after counting shipping from amazon or specialized store +whole process of measuring , mixing , applying , etc. its cumbersome
No substitute I know of that doesn't require radically opening up the hull.

Getting old white styro out is easy.. a little gasoline or acetone dissolves white styro. Pour out the goo and let it harden on cardboard.
Not sure how well that works for the 2-part poured in...

There were different makers and different foams used.... but they had one thing in common. The foam was put in before the top of the hull was bonded to the bottom. the only way i know of to get proper flotation foam back in (and add the structural stiffness that the foam added originally) is to pour in.

If it was just flotation... I'd say stuff it with pool noodles.


Upside down?
Staff member
Just to emphasize what fhhuber wrote, Signal Charlie in an earlier thread wrote:

Keep in mind that Great Stuff is water "resistant," not "closed cell" and it will soak up water if left immersed, so the boat needs to be dried out after each sail. If you want a closed cell, marine grade foam like the manufacture used, check out Fibreglast, that's what I use for repairs.
I agree with the others: avoid the Great Stuff for this purpose, it will absorb water over time and you'll be back to square one.
I bought 2-part foam from US Composites (cheapest place I could find): Urethane Foam , Expanding Marine Polyurethane Foam
I blogged about my whole process here: foam block reset
the hardest part was reaching your hand/arm into the inspection port hole and trying to reach far forward enough to get the foam everywhere.