I will recommend Cyanoacrylate adhesives.
They are sold as brand names of Loctite (sp?), Krazy Glue, Super Glue etc.
Cyanoacrylate - Wikipedia, the free encyclopedia
I use it for mending small cracks in Jnats.
They are also good to fill in some Su of a nearly Su-less Suita.
Or if you dig a 針 "needle" a small toxic inclusion.
But if your digged line is broad and deep you may not fill it all. But still it will prevent the water to penetrate under the surface layer.
Apply the adhesive 2 or 3 times to be sure it has covered all vertical sides.