Fixing small leak in the presence of water?

I have a small leak in my hot water cylinder, where the upper end of the indirect coil emerges. This appears to have a 1" BSP male thread with a fibre washer and thin backnut to seal it to the tank. The fibre washer appears to have got *spread* - probably when it was first made - and there has long been evidence of slight seapage. However, this has got worse since I disturbed the surrounding pipework for an unrelated cause.
It's all very difficult to get at due to the integral foam jacket (a little of which I have cut away) and the 1" to 22mm reducing fitting which is very close to the backnut.
If at all possible I want to fix it without any dismantling - since this would involve draining the tank - and I might even damage the coil in the process.
I have tried smearing various substances round it to seal - including plumbers mait and boss white - but these all need to be applied to a *dry* surface and won't stick properly to a wet one.
Can anyone suggest an alternative which I can use - which will cure and stick to the surrounding metal in the presence of water - maybe even *requiring* water to provoke a reaction of some kind?
TIA, Roger
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

You could try silicone. Lots of it!
It should cure on the outside, even if the inside is wet. It won't hold much pressure, though. So I hope your header tank isn't too high!
Christian.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Roger Mills wrote:

You can get a two-part epoxy "putty" that's supposed to be for exactly this situation. But I had no luck with two different brands I tried - it just wouldn't stick to wet copper in spite of the claims on the packet.
--
Laurie R



Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

I did get this to work. I think the stuff I bought was from Wickes. Be VERY careful to use rubber gloves - it's very nasty stuff. It sets as hard as metal, and worked even on damp copper.
Al
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Think a tube of Fernox LX might be what you want - it cures in the presence of water and I have had success in stopping leaks around rad valves with this.
You may need to apply it a couple of times and use a "bandage" to stop the leak pushing through whilst it's curing.
cheers Richard -- Richard Sampson
email me at richard at olifant d-ot co do-t uk
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Roger Mills wrote:

I think it's very unlikely to be a failure of the seal - cylinders seem prone to pinhole leaks, either from corrosion or from overenthusiastic fitting folding the copper into a weak spot. A weak spot can fail years after fitting, often after you disturb the pipes - as you've seen. And a tiny leak can seal itself with scale, which will drop off when you disturb the cylinder.
Unless the cylinder is pretty new, I doubt you'll get away with patching it except as a very temporary measure. It sounds like time for a new cylinder. Sorry.
This has happened to me twice before. I've attempted repairs with epoxy and Fernox LS-X. The method that came closest to working was LS-X, wrapped with strips of LS-X-soaked cloth, reinforced with twisted wire and cable ties to keep the cloth tight, more LS-X, more cloth, more wire... This reduced the problem enough to ignore it for a few days - long enough to buy and fit a new cylinder.
Be aware that if you've made a pinhole leak worse by messing with the pipework, there may be another one you haven't noticed yet...
Nick
--
Nick Shipman
Quatrain Information Limited
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Many thanks to all who replied with various ideas.
I seem to have fixed it without any fancy sealants. I managed to slacken the backnut a bit and to remove the broken fibre washer - making the leak quite a lot worse temporarily!. I then wound some ptfe tape round the thread behind the backnut, followed by a smear of plumbers mait over the top. This almost cured it, but I couldn't fully tighten the backnut because I didn't have a thin enough spanner. I could partially tighten it by tightening the 1" to 22mm reducer - but only until this bottomed on its threads.
I then had the idea of *making* a thin spanner, and looked around for a suitable piece of metal - and then remembered that I had a propane regulator spanner in my caravan. This wasn't quite big enough, but by removing some metal and putting a little bend in it, it did the job brilliantly. The backnut is now tight - and the whole thing appears to be leak-free.
Roger
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Roger Mills wrote:

That's the pioneeering spirit.
The half decent plumber who did actually produce some half decent plumbing has a car full of 'supplied washers' which he discards on every new fitting, relying on silicone rubber, hermetite, ptfe tape, hemp and mait depending.
None of his seals leak, and none of the ones I made copying his techniques leak either..

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

You _will_ remember to buy a new one before you next go caravanning, won't you? :-)
-- John Stumbles -+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ -+ The most dangerous component in a car is the nut that holds the steering wheel
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Oh yes! [The one I modified yesterday cost 69p a few years ago so, even with inflation, I should still be able to afford a new one!]
Roger
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.