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Expanding foam

Expanding foam

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Rich Griff 110/07/2021 20:50:37
40 forum posts
2 photos

I have been told that the common cheapo aerosol expanding foam can cause issues with weaker joints/ glue lines once the foam has gone off, an subject to very sunny days and temperatures, forcing joints/glue lines apart.

Is there a relatively cheap expanding foam aerosol that is temperature stable, or an alternative please.

I need to add bouancy ( seal out/displace water ) from sealed air bouancy chambers in a polystyrene double skinned boat.

Micro bubbles ?

A cheapo spray foam type material would be preferable...

Any suggestions ...?

Chris E10/07/2021 21:31:13
247 forum posts

The examples that I am aware of are not fully closed cell. If they get wet they absorb water & get heavier .

They also really do want to keep expanding and can break your model doing so.

I believe that you need something like this:

Edited By Chris E on 10/07/2021 21:34:20

Colin Bishop10/07/2021 21:33:30
4968 forum posts
6089 photos
410 articles

Stay away from expanding foam for models. It rarely ends well. There are plenty of more stable options. But if you have built the boat properly then it won't sink anyway.


Richard Simpson10/07/2021 22:40:53
649 forum posts
302 photos

The challenge with expanding foam is that it can cure at the open point of the space first, effectively sealing it, then the further expansion of the foam puts pressure on the internal surfaces of the space. Potentially leading to cracks or even failure.

Any form of closed cell sponge is as good as anything, which can be packed into a space and should never absorb moisture. It can also be removed easily for checking behind it and possible future repairs.

Just put "Closed cell foam" into Amazon, you can buy it in sheets or as a roll with an adhesive backing for such applications as door sealing.

ashley needham11/07/2021 07:24:29
7323 forum posts
156 photos

If you have a sealed buoyancy chamber it surely needs nothing else it it?? Putting anything in a sealed chamber only makes it heavier.

If you think water might get into your sealed chamber then perhaps it is not really a sealed chamber??

Ordinary expanded polystyrene is the way to go if you have doubts on the efficacy of your sealed chambers. Only sufficient is needed to stop the boat from sinking.


Rich Griff 111/07/2021 09:30:30
40 forum posts
2 photos

Thanks for the replies, I will check out the firms and places mentioned.


The boat was not made by me, but when I cleaned it yesterday water was seen "appearing" inside the hull, a double skinned affair.


Some of the seams between the inner and outer skins appear not to be sealed well enough which is why water made its way into the "void".


I will have to research liquid glues and sealers for polystyrene and flow some into the joints. Cyno is not fully waterproof I have read somewhere.


If it takes in water during a carefully cleaning with water, it will take in water when it's scooting about on the water. The "deck cover" needs some attention also as I recon water could/will end up inside the open space where the motor/batteries/radio gear lives.


All available space inside the hull open space will be filled with white expanded polystyrene "packaging waste" which is normal practise for me.


The Titanic sank.

Edited By Rich Griff 1 on 11/07/2021 09:34:56

Malcolm Frary11/07/2021 09:43:10
1029 forum posts

Apart from its knack of absorbing water, either from contact with liquid or just getting it from damp air (and boats operate in damp conditions), most users use far too much, not really allowing for it expending as much as it can.

Expanded polystyrene packing or similar, hacked to shape works well. Or similar packing noodles. Or, if you can forego the joy of popping them, bubble wrap sheets.

I imagine that the last thing wanted in a buoyancy chamber is soggy ballast that cannot be extracted and/or dried.

Richard Simpson11/07/2021 12:08:43
649 forum posts
302 photos

Check out Captain Tolley's Creeping Crack Cure:

Captain Tolley's

I recently used it to seal a clinker built hull and was very pleased with the results.

Followed by a couple of coats of Eze Kote:


Thanks to recommendations on here!

ashley needham12/07/2021 16:16:32
7323 forum posts
156 photos

The only time I have used expanding foam was when I had a water issue in the carpet carrier (as the deck just fitted over the hull and water was coming in the bow).

SO I squirted foam in the front compartment....BUT this didn’t matter as the top was of course open, so I left it a few days and then trimmed it flush. Water still gets under the fore deck but runs off a bit further on off t( foam.

Absolutely would use it for anything enclosed.


carpet carrier#4

phill downes12/07/2021 21:22:32
14 forum posts
12 photos

ok here is the thing with spray foam and 2 part foam.. first the spray foam.. if the cavity is large it will not go hard in the middle when you spray it. ive got a boat I done this to 4 years ago and in the middle is still soft and gooey.

also you need to make sure that you do NOT seal the area until the foam is fully cured unless you are after trying to make the double skin stronger. be very careful as there is power behind the foam when its curing /expanding.. to be honest its rubbish.

2 part foam you mix is also pretty dodgy to use as it expands a lot, and I mean a lot and if you try to stop the expansion with a plug it will blow your polystyrene to bits...

here is the downside to any of the expanding foams.

they are water tight all the time the skin on the foam is unbroken... BUT as soon as there is the slightest opening of the skin then its rubbish as will let water in and you cannot get it out unless you squeeze it under high pressure.

you cannot just let it dry out, that dont work... I had to tear into a 13ft dory and completely take the boat apart to get rid of so called water proof 2 part foam. took me 3 weeks working 6 hours a day to get the boat apart and then clean out the foam..

the best thing you can do if you need flotation is to buy pool noodles for swimming pools etc. long tubes about 4ft long and cut them up and stuff them in, they will NOT allow water into the foam and are brilliant for using in a model boat for flotation and they are cheap...another thing is ping pong balls.

I use 3 complete noodles in a 5ft powerboat and it will not sink even when filled water, I tried it......

for kids 1 pool needle will keep about 5 stone above water., actual dead weight I would think about 10 pounds per noodle.

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