Don't use greatstuff as a sealant. It sucks at doing anything near that... It is safe to use as a background however. Dart froggers have been using it for years.

To make a non shred-able background do the following:

1. Lay down layer of greatstuff for background.
2. Let cure for about a week to be safe.
3. Carve to your liking. This step isn't very important.
4. To make the background unable to be shredded and look like rock, you must cover it in a hard rock-like substance. Purchase a bag of tile grout of your desired color, or concrete dyes to change its color.
5. Next you must get the grout at least semi-sealed before it can be used. When mixing your grout to apply to The background, instead of water use Quikrete Acrylic concrete fortifier. You want the consistency to be like runny dough.
6. Apply sparingly (just enough to cover the foam) and shape as needed. This will cure with a fan in about 4-5 days. This method can make truly stunning backgrounds.

Sorry I posted a while after your initial post :

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