Loulou57 is correct, at least in part. First you have to stop the crack from generating any more. You do that by drilling a hole, probably 1/8" is enough, 3/16" better, at each end. The sharp end of the crack is an infinite stress riser, the round hole reduces it tremendously. As far as sealing the crack the believers in the 3M products are probably right. Personally I'd use some of my boat building epoxy as that's what I'm familiar with and have. But first you have to stop the crack from getting any longer.