You didn't say what material the kayak is made out of.
Kayaks can have many different types of finish.
Poly resin fiberglass and plastic rotomolded are the most common nowadays.
Other common types include epoxy resin, fiberglass, kevlar and/or carbon fiber mixtures, some with gelcoat some with other finishes.
Wood kayaks are usually a varnish coat over epoxy/fiberglass.
Skin kayaks can be nylon, polyester, cotton canvas, ... coated with hundreds of different types coatings.
What ever you use should match or at least use a coat that works with the original coating.
Epoxy can be used to repair poly resin kayaks fiberglass kayaks.
In general, you want to put gelcoat over any epoxy or poly resin as they are susceptible to UV damage.
And I'm keeping it simple:-)