Leaking sink waste connection ?

I have just fitted a new sink in my cloakroom which has one of these click/clack waste fittings, with the overflow running into the slotted waste fitting The plug fitting drops through the hole in the sink, with a plastic washer between the plug and sink, with another plastic washer/seal that fits under the sink with a large nut to tighten the plug to sink joint. My problem is I cannot tighten the nut enough to prevent water running down the thread of the waste fitting below the sink. I'm guessing that the water is finding a route either between the plug and sink on the top side, or possibly through the lower washer ? Is there a better way of sealing this ?
Thanks Tony
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Tonyb wrote:

Buy a tube of Fernox LS-X, apply thinly to all mating surfaces, leave for an hour.
--
Dave - The Medway Handyman
www.medwayhandyman.co.uk
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wrote:

click/clack ..I presume you mean a popup waste . I have one of those and had no problem with any leakage but I think I dumped the upper washer and used Plumbers Mait to seat it .. As for the large nut underneath I have just had a quick look using a mirror and there is a washer underneath but I have a feeling it is rubber not plastic .I think I might have smeared it with LSX sealant and certainly I have had absolutely no problem with it since it was fitted. Are both the washers you got with the waste plastic ?...is one not rubber .
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Concentrate on sealing behind the nut.
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The lower one is rubber. So the general consensus seems to be to use fernox lsx sealant/plumbers mait and possibly not use the upper plastic washer either ?
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... or silicone

what goes on on the upper side makes no difference re leaks. Plastic washers just spread the load.
NT
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The washer between the sink and the plug stops the water draining when the plug is down - leave it out and the sink won't hold water, although it won't cause a leak.
I prefer using these http://www.toolstation.com/shop/Plumbing/Washers+Fixings/Basin+Mate+1+14/d20/sd2703/p37534 on the underside rather than using sealant.
A
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