GREAT STUFF FOAM QUESTION.

Hey all Im considering laying down a floor in my jon boat and I plan on using that great stuff expanding foam for my base between the ribs or the boat. Then laying water sealed 1/4" plywood over it . I am mainly using that spray in foam because i have 11 cans of it in my shed unopened and i am on a very tight budget. Any suggestions or tips would be greatly apreciated thanks Tin boat members.

Great Stuff will absorb water unless you paint it, this is in the directions on the can. Use a couple of coats of paint because the first coat will absorb into the foam just like water will.

Will do I would imagine let the foam cure real good before I put a coat of paint on right? OR maybe use Thompsons water sealer on it that may work better than paint ?

How are you gonna paint the bottom of the foam? You need a closed cell foam - Great Stuff is not boat flotation foam

Im not looking for floatation purpose more for supporting the weight when we walk around on the plywood. Thus using the foam to fill the void btween the wood and the hull of the boat sorry I didnt specify this earlier.

Again, you're making a BIG freakin' mistake if you use that not-so-great stuff. It absorbs water. Not only will this cause your boat to take on weight, the waterlogged foam being in contact with your aluminum hull will cause a condition known as 'poultice corrosion'

If you want to do it right and not ever have to worry about it, get some Dow foam board, either blue or pink, and place that under your flooring. The foam board is closed cell and will not absorb water, it is also rigid and will add strength to your floor, as well as dampening vibration.

Despite the fact that expandable foam soaks up water boat builders still use it everyday. Explain that one lol.

PSG-1 said:

Again, you're making a BIG freakin' mistake if you use that not-so-great stuff. It absorbs water. Not only will this cause your boat to take on weight, the waterlogged foam being in contact with your aluminum hull will cause a condition known as 'poultice corrosion'

If you want to do it right and not ever have to worry about it, get some Dow foam board, either blue or pink, and place that under your flooring. The foam board is closed cell and will not absorb water, it is also rigid and will add strength to your floor, as well as dampening vibration.

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+10. Totally agree. Please don't make this mistake.

Xpress_442 said:

Despite the fact that expandable foam soaks up water boat builders still use it everyday. Explain that one lol.

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Just like board foam comes in closed and open cell varieties, expanding foam comes in both types. Any decent boat mfg is going to use closed cell expanding foam. Great Stuff is "NOT" closed cell, and therefore not suitable for use in marine applications.

If you keep your boat covered or garaged when not in use you won't have any problems.

If you seal the foam it'll protect it in case you get caught in a rain, or when you wash your boat out.

You need to keep your boat covered anyway if you're going to use plywood, the cheap $20 tarps at Wal-mart work great for boat covers... it's what I use.

If you MUST use it...in spite of the advice given above...

I'd suggest that you put large doubled-up plastic bags down in the area first. Then spray the spray foam into the bags. Then, after you've done the deed, spray (probably latex) paint onto the foam while it is still inside of the bag. Do a trial, because many spray paints (actually their propellant) will dissolve foam.

There is a brand of latex spray paint that is supposed to be foam safe. For a model airplane project, I bought it at a hobby store, Micheal's or Hobby lobby, I don't remember which. Pretty expensive per can.

Seal the bags as best you can, perhaps by using double sided tape, folded over and re-taped a couple of times. Squeeze all of the air out that you can. You don't want a bag-bubble bursting the first time you step on the deck.

That way you will have, at least, a modest water barrier in place.

regards, Rich

Good advice, rich, and that's pretty much how the factory does it, too. 8)

If you look at a factory built boat, you may see plastic plugs on the decks, etc. This is where the factory inserts a plastic bag, where they give it a quick inflation to make it fill the space, then, they pour in the 2-part foam, except, the foam they use is closed-cell, it's not like the 'great stuff'

I know about using that spray foam in a boat, when I re-did my jetboat, I had the pleasure of digging out water-logged foam from around the fuel tanks. The waterlogged foam also caused corrosion issues with the aluminum tanks, heavy pitting, which, I took care of the worst of it by the tedious job of TIG welding anywhere the pitting looked deeper than about .025"

Fortunately, it did not damage the boat's hull, because it was painted with truck bed liner, and there was a sheet of 1/2" dow foam board placed under the fuel tanks before the spray foam was blown in. But, the foam board being in contact with the bottom of the fuel tank also caused corrosion, so, this time, I didn't put any foam under it, for good ventilation and drainage, and the tanks are elevated about 1 inch off the bottom of the boat with ribs, and those ribs are topped with a strip of industrial-type rubber, with the tank's brackets bolted down to that.

Thanks guys I decided not to use that xpanding foam. I am going to wait till I get some money and try to buy some foam to put in. Unless I can find soem for free lol

Swampthing(True) said:

Thanks guys I decided not to use that xpanding foam. I am going to wait till I get some money and try to buy some foam to put in. Unless I can find soem for free lol

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If you know of any contractors in your area that build docks, you might be able to buy some billets of dock foam from them, at a cheap price. Specifically, a damaged billet, like one that's been broke in half, or maybe some drop material where they had to trim billets to size for a dock. The only thing is, then you would need to cut it down to the size you need.

Just a thought....

4 x 8 sheets of foam sheathing go four about $10 a sheet at the big box retailers around me.