I am a consultant and I am providing engineering / material science services for my client who is in need of sealing a water leak in a forestry piping system.  The system pumps clean and filtered water from a lake.  In an unpressurized section of PVC pipe (outside / exterior location), a leak has developed.  I can either remove the pipe sections, break apart the concrete footings and supports, then re-install a new section of pipe and valves/Forged Steel Valves - VERY EXPENSIVE !!! Or, I can inject a sealant into the pipe from an opening above, which would (hopefully) seal the water leak for good.  

I am looking for a sealant (or adhesive) which I can inject down into the pipe section (about 6 inches down) which would cure underwater to positively seal the low pressure water leak.  The water temp. in the summer is around 70 deg. F.  The water temp. in the winter is above 40 deg. F.  I'm trying to figure out the best method of injecting this sealant.  My first impulse is to pour the sealant into a metal cylinder (like a grease gun), then inject the sealant (using an air compressor) to force the sealant to it's desired location.      

QUESTION:  WHAT TYPE OF SEALANT CAN I USE ???  WHAT TYPES OF SEALANTS WOULD CURE UNDERWATER ?  WHERE WOULD I PURCHASE THIS SEALANT ?  DO YOU KNOW OF ANY COMPANIES THAT CAN HELP ME OUT ?

Silicone should seal just fine underwater. Try GE. They make the stuff in all sorts of viscosities. How about using a silicone rubber patch with silicone sealant applied to it. Hold it in place with hose clamps.

I know of two sealants used on boats that will cure under water.

There are epoxies that will cure under water, but in order to get them to stick to PVC, you need to first apply some PCV solvent.  Even then, I'm not sure that you'll get the best bond.  You can contact the Gougeon Brothers, the makers of West Systems Epoxies.  They are very helpful and knowledgible.

3M also makes sealants for boats and of course are VERY knowledgible.  I can't remember if it's Polysulphide or Polyurephane (probably polysulphide) that NEEDS water to cure.  I believe that the product names are 4200 and 5200.  A 3M representitive can probably help you.

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