# How long does Great Stuff continue to expand?



## nixie

Well, I pretty much summed it up in the title. I formed these mushrooms a week ago using great stuff, waited a while, coated them with foundation coating, and they keep forming little cracks with great stuff squeezing out! GRR! Im trying to wrap things up here...
When will this stuff stop expanding?


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## The Watcher

It depends on how thick it is. I made a shark last year well it still expands some. I don't know if it ever shrinks any because of weather or moisture. But I had the cord set in it's mouth, when I got it from the attic. I went to check it out. I had to get a 2x2 and push the foam down to get the cord out.


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## nixie

Oh no! These things are good 2 1/2 foot thick!


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## morbidmike

your scewed I just did my new demon head it expanded about 3 times the size I wanted


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## Spartan005

funny, the stuff I got was maybe twice its original size by the time it finished expanding and that was it


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## Devils Chariot

great stuff is urethane foam with is hygroscopic, which means it absorbs moisture from the air, which is part of how it expands. I just looked at a guy I made from all great stuff last year. I painted him weeks after I shot the foam, and now a year later all the paint has cracked all over.


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## debbie5

They should combine it with Viagra. Now THAT'S a product to invest money in.


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## nixie

I think the problem is that every time I put another coat on it, I'm adding moisture, thus making the problem worse...
Deb-lol!


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## The Creepster

Ahhh great stuff is great for stuff...right?


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## NecroBones

Great stuff expands a lot in the first 20 minutes, a continues to expand gradually for something like 24 hours. I usually wait a few of days before painting it. It'll look great for months, but 6 to 10 months later the paint will almost certainly show some splitting, and gaps pulling open between the "lumps".


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## Lord Homicide

Old topic but which Great Stuff foam expands the most? Has anyone ever tested this? I want to make a foam dome (using a large salad bowl from Dollar Tree) and use the least amount of foam


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## Troll Wizard

Lord Homicide said:


> Old topic but which Great Stuff foam expands the most? Has anyone ever tested this? I want to make a foam dome (using a large salad bowl from Dollar Tree) and use the least amount of foam


I have always used the regular Great Stuff foam. It does seem to work best when you are using a mold, like your salad bowl for your dome. I experimented with several different ways of spraying it and found out that by putting a thin smooth layer of petroleum jelly on the inside of what your mold is keeps it from sticking to the mold itself.

Okay you do have to wipe it off afterwards but it has always seem to work for me. You might also want to put something like a heavy flat board to keep the expansion down when you get to the top of the bowl. That way you might keep the bubbling down.

I haven't been able to keep the foam from cracking over time as it dries, but I have use caulking to fill in the cracks and it makes it a more uniformed look. If the cracks are really deep, then you drying time for the caulk will be longer as well.

As far as painting is concerned you might want to use an oil based primer first then paint after it's dry. With an oil primer you can paint latex over the top of it or other types of paint such as acrylics. I think the primer will adhere to the surface of the foam better. You might spray out a small amount of foam and paint it with the primer just to experiment with it. You can get oil based primers in spray cans.

Just remember the rules of paint: 
1. You can always put an oil based product (primer or topcoat) over a latex (water base) topcoat.

2. You can't put a latex topcoat directly over a oil based topcoat (but you can put it on over a oil primer). The latex topcoat will peel off over time if painted directly on a oil topcoat. Oils have a natural sealant in them on their surface that keeps water based products from sticking to them.

3. Oil based primers work the best, because your using foam which has an oily base substance already it's best to use an oil based primer so it will stick better. Plus primers makes your colors stand out better when painting on topcoats, and it keeps your topcoats from being absorbed into the surface of the item your painting.
:jol:


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## Bone Dancer

Thickness seems to be the main issue. I tried to do a skull and the middle didnt cure and it later sunk in. A friend suggest that I place something inside the skull like a pvc tube that was well greased with some kind of mold release ( oil, car wax, ect) and to remove it as soon as it starts to cure a bit. These would leave an open area to allow you to spray water and let the middle of the GS cure.
I have not tried this yet but it sounds good to me.


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## Lord Homicide

Damn, I was hoping to avoid an armature.


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## Troll Wizard

Don't know it this will help you but Great Stuff list this in their FAQ's for how much is in a can of Great Stuff...

The theoretical yield of one-component foams is measured in lineal feet of 3/8" and 1/2" diameter beads. Gaps and Cracks - 12 oz. 259 lin. ft. at 3/8" and 146 lin. ft. at 1/2" Gaps and Cracks - 16 oz. 328 lin. ft. at 3/8" and 185 lin. ft. at 1/2" Gaps and Cracks - 20 oz. 417 lin. ft. . at 3/8" and 235 lin. ft. at 1/2"

It also list the tack and drying times: 

GREAT STUFF™ is tack-free within 5-10 minutes and cures fully in eight hours. The higher the humidity, the faster the foam will cure. Misting a layer of water on the surface prior to dispensing the foam will speed up the curing process.

Don' know if this will help you in figuring out how much it will expand?


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## Lord Homicide

Thanks Troll... every little bit helps. I bought a couple of salad bowls at the dollar store and just sprayed the foam into the bowl hoping that it will expand into the dome I'm looking for.


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