coolant tank repair

[Wolfsbrg8v at aol.com (Wolfsbrg8v@aol.com)]

Anybody know what kind of sealant or glue will seal 
a small (1/2", hairline) crack in the coolant tank?

I've tried marine epoxy, HP-66 cement, tire cement, 
Duco, and nothing seems to want to stick to this plastic.

It has to be sealed from the outside, as I cannot see 
or reach the crack from the filler cap hole to get at it 
from inside.

Any advice would be much appreciated, I'm tired of
these green puddles under my car!

Leo