A toughened, versatile epoxy that has been pre-thickened for permanent waterproof binding of fiberglass, ceramics, metals, plastics, and damp and difficult-to-bond woods.  It's got a little more flex than standard epoxies but is much stiffer than adhesive sealants, so it makes great structural bonds that can absorb the stress of expansion/contraction and vibration. 

You can use West System fillers and additives with it.  G/Flex gives you 45 minutes of working time at room temperature when mixed at a 1:1 ratio and reaches a full cure in 24 hours.