I've installed an inspection cover on my fresh water tank to allow cleaning but I am having trouble making a good seal. The tank is rigid plastic and I have tried silicon sealant, but it leaks. Can I use sikaflex safely?
Every inspection port retro-fit kit I've ever seen has a rubber gasket that goes under the flange (which is attached to the tank with screws)to seal it to the tank, and a rubber o-ring to seal the threaded cap.
If yours is missing the rubber gasket, make one. And get a nice thick rubber o-ring for the cap. Rubber gasket material is available from most hardware stores here--so I'd assume the same is true in the UK. It'll give you a much better seal than any sealant.
I agree with the rubber seal option. I would not use sikaflex as is sets so hard that you may damage the cover when you need to remove it. On a similar application I used a rubber seal with silicon as well and it worked fine.
I agree Sikaflex is too adhesive! However a very cheap source of gasket
is bike inner tubes! I am about to reseal my windows with this abundant material.
You will only need the slightest smear of sealant, if any.