There's a two part epoxy paste called splash zone that I have used on 3 pipes and still holding. Its designed for marine 
applications, in particular, to repair the hulls of vessels at the "splash zone" of the bow, while the boat is floating. When 
I first heard this I laughed..... Yeah right, apply this to wet wood or fiberglass? No way, never stick!
Then working at the Santa Barbara harbor servicing one of N.O.A.A. boats, (my neighbor was the captain) when this man 
asks if I had any free time, to do some things to his boat. When I finally got to it, he had a diesel tank leaking on a corner 
where it was welded. Not me, Youll need to have this welded. The owner broke out some thin rubber gloves, a bucket of 
water, and two quart size cans. One had black tar in it, the other had yellow tar. I thought he was nuts. but he's paying me 
by the hour, so whatever.
The procedure: gloves on, scoop out equal amounts of the black and yellow tar, and mix together with hands, in the bucket
of water until blended into one solid color, and its ready for use. but the diesel tank is seeping fuel out the crack, this is a joke!
So after building up the splash zone over the crack, I'm told to wet my hands and smooth it out.
The next day it was no longer leaking. I worked there for 2 weeks, checking it every day, but still no leaks. Amazing!
I went and bought it in gallon cans for $300. fixed the aluminum core plugs that rotted out easily, without removing the pipe.
I used a wire wheel to remove paint and flaky shit, and smeared it on, smoothed it out, leaving it a min of 1/4" thick.
The guy told me it will fix a boast hull underwater, that's what it was made for!    SPLASH ZONE !