Plumbers putty is pretty much rock-hard forever, I wouldn't be surprised if it wsa the same thing as the marine products. For a temporary leak stop, it should last very nicely. it is meant to seal damaged pipes--permanently--in places like wood frame walls where you don't want to use a torch to make a proper repair.
CB, now you understand why gate valves are considered uninsurable by many companies. They're not suitable for anything beyond garden hoses. Beware the ball valves at Home Depot and such, they are usually brass (not bronze) with a stainless (not bronze) ball in them, and they can rot out from galvanic action in a boat surprisingly quickly. I'd leave the epoxy where it is, and if you can stand a week without sailing, plug the raw wate intake from the outside with a gob of beeswax (sold for $4 as a toilet bowl ring seal, warm it up before gobbing it in) as insurance.
Then, order a proper BRONZE or MARELON (not just plastic, but Marelon) seacock or ballcock on install on the fitting, and do the repair just once. Temporary repairs to these things tend to be too convenient--and then two or three years later you say "Oh, yeah, I meant to get around to that..." when it finally does fail again. Better to do it once, and do it right.
If you have any other gate valves on the boat? Order the replacements at the same time, put them in when you can. And, order a couple of sets of tampons (aka bass wood damage control plugs) and attach one with a string (means drilling a 1/4" hole through the top of each one) to each fitting that might fail and need to be plugged. Cheap insurance--and good seamanship. You tie them on with a foot or so of string, so there's no fumbling about if you need to find one while there's water gushing in.
Last edited by hellosailor; 09-20-2008 at 10:29 PM.