Tricky Plastic Tank Repair.... please help.

[Papawads]

Have finally discovered the source of a fresh water leak, an odd one too. My main water tank (250L) has a tiny crack, about 1/3 way up from base, which only leaks when it's full; presumably due to pressure.
It's taken me months to find it, draining what little faith the Admiral had in my technical abilities.
The tank is now out, but I'm foxed about how to repair her. The best way to picture the location of the tiny tear, is to imagine a step on a staircase. To one side, is the point where the top of the vertical riser meets the start of the horizontal tread (step) AND the vertical side wall.
Any view on what to do with this would be most welcome. The tank doesn't fit through any door, so it's currently upside down on the salon table, with a dark cloud sat brooding behind it....
Papawads
:-)

[GordMay]

If your water tank is thermoplastic* [polyethylene, polyurethane, etc] Google “Welding Thermoplastic”. [eg: 1]

In order to get a strong, consistent weld, it is necessary to make sure your substrate and your welding rod are identical; for instance, polypropylene to polypropylene, polyurethane to polyurethane, or polyethylene to polyethylene.
The most important rule about welding polyethylene is that you can weld low to high, but not high to low. Meaning, you can weld low density polyethylene (LDPE) welding rod to high density polyethylene (HDPE) sheet, but not vice versa. The reason being is quite simple. The higher the density the more difficult it is to break down the components to weld. If the components cannot be broken down at the same rate then they cannot join together properly.

* A "thermoplastic", means that it can be melted and reshaped with heat. When you weld PE/PU etc, you perform a fusion weld, meaning that you melt the welding rod and base material together.

[1] ➥ https://waterwelders.com/how-to-weld-plastics/

[peghall]

Heat welding is the only way to repair a crack in a polyethylene tank and just shooting some melted PE into the crack won't work. It's necessary to melt the edges of the crack too..enough of the material to let the new PE to blend with it so that they become a single mass. It requires a bit of skill to know how much of the crack to melt to achieve that., so if you can remove the tank to take it to a plastics fabricator, that's what I recommend. Fortunately it's a water tank...it's almost impossible to find anyone who'll work on a "used" holding tank. If you're lucky you might even find one who'll make a "housecall" to fix it for reasonable price.


--Peggie