I was looking at some "Great Stuff" expanding foam at Lowes. It says on the can that it "provides a permanent air and waterproof bond". It recommends painting exposed surfaces. So, if I paint it, will it keep water out?
I was hoping someone in the know would have a definitive answer for you. I don't have that so I'll share what I did and you can draw your own conclusion. Nine years ago I removed a double window unit and replaced it with a set of French doors. The exterior of our house is brick and the saw I was using would not cut all the way through the brick leaving about and inch that needed to be chipped away. Most of the brick I was able to break off pretty clean but two broke up leaving a gap that was about an inch wide and eight inches long. I filled that gap with foam, cut it flush and painted it. It is still there, hasn't pulled away from the brick or molding and has held the paint well. As far as I can tell it is waterproof, at least in this application.
I think the application the foam is used for would be the variable that determines whether it would work as a waterproofing. The foam never hardens and could easily be punctured. If those are not issues, my experience has been it works and is waterproof.
There are different kinds of foam. First, there are several different expansion levels, from almost no expansion to blow your project apart. There is even one that minimally expands but remains soft and is for use around windows and doors. (Guaranteed not to deform door and window casings).
Waterproof depends on the brand and type of foam. Some foams are now latex and clean up is with soap and water. When we were building, I used about 3 million cans of spray foam. I was buying them a dozen at a time. For outside work I always made sure I was using a foam that said waterproof bond, or did not have a soap and water clean up. There was one particular brand that I was getting that said it was for outdoor use, but I can't remember the name.
The guys are right, the "painting" refers to sun exposure. Most foams will degrade in the sunlight. I believe this is engineered into the products so that we don't have little bits of foam blowing everywhere for a hundred years. But, it makes it hard to maintain the foam outside, because it will brown and begin to turn to dust. When not exposed to sunlight, the foam will last many, many years without changing.
Hope this helps. What are you trying to use it for?
The folks who used to own my house (Inspector507 calls them the Dreaded Previous Owner aka DPO) left a gap about 2" wide between the house roof and the breezeway roof. The house roof is about 2' higher and overhangs the breezeway roof about 18" so most of the time it's OK, but if the wind is in the right direction water will get through. I want to use the expanding foam to fill this gap. I thought of using flashing, but the expanding foam would be cheaper, easier, and faster. It won't be visible from the ground, so it doesn't have to look great.
It sounds like it would work fine. The foam isn't structural, but it will do a great job of keeping out the wind. I prefer a minimal expansion foam since the tendency is to use too much and the high expansion stuff can easily get out of hand. Also, the high expansion seems more prone to air bubbles.
When you're done filling, let the foam dry. Then, if you have room to work, get one of those sanding disks that fits in your drill. I prefer the kind that looks like it's made out of a furnace filter with grit glued on. It will help you smooth the foam down for painting, etc. You can run right along a board and grind the foam to the same heighth as the board with no problem. (Be sure to wear a dust mask, the dust will leave you feeling off for a few days).
If you have that big of a space, you will need several cans of foam, whether it is high expansion or not. The waterproof bond stuff should work. If you want, you could leave the surface rough for now and grind it flat in the spring, when the weather is more conducive to painting.
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