What you see in that pic is epoxy mixed with microbaloons and cabosil to help set the stab in place.

The wing halves are joined in a similar manner. I mix cabosil and milled fiber together along with a slow set epoxy (usually 24 hr cure, 45-60 min working time) when I join the wing halves. The mix is applied with a large syringe as a bead around the wing and joint areas.

Michael