I’ve tried to fix a few plastic tanks and it seems, with some, they lose their plasticity over time. After a certain period any pressure causes them to crack so they don’t lend themselves to repair. It tends to be the semi-translucent unstabilised plastic (HDPE) that suffers the most from this; the type radiator overflow, windscreen washer liquid and some inexpensive mower fuel tanks are made from. It can become really brittle and if you glue it in one spot it breaks in another. With these the only solution is replacement.
The black plastic tanks (ABS) tend to last almost indefinitely and only really suffer damage through some form of misadventure. I’ve had taps snap off on Victa plastic tanks because someone has been a bit rough. I’m hoping your friend may have had this happen as it’s just a matter of manipulating the remaining stub of the tap out of the base of the tank and bunging in a new fuel tap.
Otherwise with ABS you can drill and tap it to affect a repair. I’ve even seen ABS parts “plastic welded” using a soldering iron, a strip of heat resistant fabric and a little more ABS as filler. This was done when a water tank cracked at a seam. The bloke who did it wasn’t working in anything like an ideal environment and was just using improvised tools and the repair held up really well.
