advice on mending broken plastic seat on kayak

hi everyone im new here and defo need some good advice recently i bought an old sit in kayak pretty old the seat is plastic and needs bonded to what i believe is fibreglass as the seat is very much broken off can anyone suggest a really strong adhesive that may do the job ? all suggestions greatly welcomed (its my first kayak ) thankyou in advance

 

Use the seat as a mold and make a new one of fiberglass using epoxy resin. The epoxy won't stick to the plastic so when it has set up you can just peel it off.

 

Oxidize the surface with a torch and use GFlex epoxy, from West System on the parts. This will bond fairly well, but unfortunately nothing really bonds well to those types of plastics. Log onto Westsystem.com and look in their user's guide for difficult bonding applications. With any luck they'll have updated the site to include the newer products and techniques.

 

What type of plastic is it? Fiberglass? Polyethylene? A picture would help.

 

Buy a new seat. It's the best long term solution. To fix a polyethelene seat will cost more (if your time is worth anything at all) and it won't have a lifespan as long as a new one.
That said, I've repaired polyethelene by welding it using a hot knife (my tile saw reservoir has lasted a year through a few tile jobs with no leaks yet).
Adding somw fresh plastic makes the joint thicker and stronger. Some could maybe be cut from the edge of the seat. You could heat it in a metal can on the stove (carefully, not too hot) and pour it into the fresh hot weld, or have a helper follow your hot knife as it melts a groove into the crack, pouring it in before the groove cools.