Stopping a (top) leak to my HDPE water tank

[Modulation]

The tank used to have a fairly useless gauge at the top and the rigid plastic screwed down cover plate leaked heavily if the tank was full and the boat heeled. The opening is about 60mm dia and the cover plate 90 mm dia.
Surmising that the problem was that the 6 screws were not capable of being tightened enough, I replaced them with bolts so that I could bolt the plate on firmly using silicon as a gasketing material and to seal the bolt heads. I chucked away the gauge. Fortunately, the large screwed access panel to the tank is just beside the opening.
All ok for about 4 weeks but then the leaks restarted. And now there as bad as ever. Grrrrr.!
Any ideas for a permanent solution?
Thanks

 

[David2452]

You really need to "mirror" the panel bolt PCD inside the tank with a rigid ring, it will need a small section removing to insert it, or be done in two halves. You could have a patch hot air welded but then you would have no access.

 

[Talulah]

When you used silicon as a gasket material do you mean silicon out of a tube or silcon sheet. Silcone out of a tube would become too thin as the bolts are done up. Instead use some thick silicone sheet. Cutting up a silicone place mat would do. Alternatively send me a pm and I'll send you a suitable piece.

 

[nigelmercier]

Silcone out of a tube would become too thin as the bolts are done up...

So tighten the bolts until there is a 3mm gap, allow to cure, then nip up a bit.

 

[Talulah]

So tighten the bolts until there is a 3mm gap, allow to cure, then nip up a bit.

Well that's an option of course. I don't know about you but I prefer to get the whole job done in 30 minutes. Not have to revisit it the next day. It's the same problem people have resealing windows. They squeeze out all the silicon and then wonder why they leak soon afterwards.
When I improvised a blank for a holding tank I cut up an HDPE chopping board and a silicon plate mat. Worked perfectly.

The other curse of course is opening a tube of silcone. It will have gone off by the time you want to use it again.

 

[MoodySabre]

If you look at a Tek Tanks tank then the hatch has 12 fixings and a gasket. Perhaps it distorts without close fixings

http://tektankslimited.com/ekmps/sh...rtical-waste-tank-2018-p[ekm]175x217[ekm].jpg

 

[alahol2]

I would have thought that a piece of closed cell neoprene sheet would be soft enough to compress but springy enough to seal a 60mm hole.

 

[ean_p]

as an aside...does anyone know where to buy the weld in fittings for tanks like screwed sockets etc please?

 

[David2452]

If you look at a Tek Tanks tank then the hatch has 12 fixings and a gasket. Perhaps it distorts without close fixings

http://tektankslimited.com/ekmps/sh...rtical-waste-tank-2018-p[ekm]175x217[ekm].jpg

It does, that's why I suggested a stiffening ring in the tank, even the 25mm hole for heater standpipes can be a nightmare to keep sealed, but I have at last (after having to have people come and hot air weld bosses on and using all sorts of sealants with only temporary success) found a reliable readymade solution in this http://www.aquafax.co.uk/html/product_specification.asp?ID=23439 it is designed for 1 1/4" BSP fittings but serves well with a reducing bush.

 

[David2452]

as an aside...does anyone know where to buy the weld in fittings for tanks like screwed sockets etc please?

pages 8 & 9 of Tek Tanks catalogue.

 

[theguerns]

I had the same problem with our Colvic Watson 32. The fuel and water tanks have 3 inspection plates in them and when full leaked. In the end I used a product called CT1 (toolsandsealantsdirect.com) resealed them and filled them and after 2 years we still have no leaks. I have used this sealant on skin fittings,deck fittings and general sealing on the boat and have found it to be the best on the market. It kicks Sikaflex into a top hat.