Marine-Tex is the one for me.

I buy it at a place in Palo Alto, Ca called West Marine.

Comes in Grey or White.

Same as another product mentioned, works underwater, fixes cracks in cylinder heads, and will repair gas tanks.

I just used it to savemyself $450 for a Honda radiator with a cracked fill tube.  Fixed, done, perfect!

Kim Springer
'89 Tristar 2WD "#7"  White