sealing a sensor in a plastic water tank

[Birdseye]

Bilge full of fresh water yesterday - turned out to be the tank guage sensor on the plastic water tank when the tank was full to the brim. The sensor is a hard white plastic - the tube itself might be abs, but the top part that sits on top of the tank is pretty hard, bit like a white light fitting. The tank itself is a greasy feeling black plastic from the 90's so probably polythene.

Now its clear that the previous owner had trouble with this fitting - it was sealed to the tank top with silicone sealant which hadnt bonded in any way with the tank itself. The tank top is uneven so a physical gasket wont do the job.

Any chemical engineers here who know how to glue plastic to plastic?

Failing that the only thing I can think of is to rebed onto silicone, let it harden and then screw down all roung the fitting to compress the silicone gasket.

Suggestions welcome. How would it be done by a proper boat building company?

 

[oldsaltoz]

grab yourself a white lid off an ice cream container >PP< stamped in it (PolyPropylene) cut into strips about 8mm wide, heat with a soldering iron and form a weld.

Practice welding up a split in the ice cream container first.

You can also use the above to relocate the fitting, remove fitting, cut a circle out of the ice cream container and make a small hole in the middle, put a small wire hook thru' the hole and push the circle inside the tank and back up to cover the hole, weld then remove wire and weld over the small hole, now cut another circle and weld on top of the first one, this will form a double sided patch.

Avagoodweekend......

 

[Matrosen]

Your description of the feel of the tank makes me think it is propylene. If so, the weld is the only way to go, as nothing sticks to it.

 

[Robin2]

I don't believe it is possible to weld two different plastics together. I would either use your own silicone suggestion or get a soft rubber washer of the sort you get in screw-together waste water pipe fittings. An o-ring might be another possibility - just used as a sealing washer.

 

[srp]

A recent post described how to make your own gasket using Sikaflex, with cling film to prevent it sticking. Wait until its cured, remove the cling film and you've got your gasket/sealing washer.

 

[pampas]

Try Red Stag "B" used by engineers the world over for sealing most problems, do not use the "A" grade as this is a fine texture whereas "B" is course and the grits will make micron indentions and should seal ok.

 

[billskip]

In some cases silicone and other sealnts do more harm than good.

IMHO clean every thing off, if all is in good condition, make a dry connection using rubber washers (the type you can get from plumbers merchants for sink and bath waste)