Sealing hole in holding tank

[ianc1200]

My black water holding tanks - look like Vetus but no manufacturer's marks - have a sensor about half way up the vertical face of the tanks. I have removed these, about 1/2" diameter, how best to seal up?

 

[johnphilip]

put it back?

 

[PaulRainbow]

My black water holding tanks - look like Vetus but no manufacturer's marks - have a sensor about half way up the vertical face of the tanks. I have removed these, about 1/2" diameter, how best to seal up?

If you have access to the inside of the tank, a nut and bolt with big washers and plenty of sealant. If not, a small plate, plenty of sealant and self tapping screws or pop-rivets.

 

[mrming]

Never has sealing a hole properly been so important.

 

[ianc1200]

Thanks for answers. BTW Paul, did buy the 2nd hand davits you previosly mentioned from Mark/Mr Stainless.

 

[Ian_Edwards]

If it's a Vetus polypropylene tank, you can weld a plug in.
I've successfully patched polypropylene water and diesel tanks using offcuts of polypropylene, with a temperature controlled soldering iron and polypropylene welding rods bought from eBay.
You can buy Polypropylene weld kit on eBay if you want a more professional weld.
It took a bit of practice the get the iron at the right temperature.
Cut a disk a little larger than the correct diameter, so it doesn't drop though the hole. I chamfered the edge of the disk, so that it "sat" in the hole, then carefully tacked it at 3 points, before welding the circumference.
The repair is still sound many years later.