When building a boat from plywood, you need to glue and tape the the corners and edges of it with fiber mat and epoxy resin.
Fiber mat and Epoxy however, don't like sharp corners and edges. That means you have fill up the corners to make them soft and round and you will usually do that with some Marine epoxy as a Filler.
The only problem is that the Filler must be still wet and soft when you place the fiber mat and the epoxy on it to let it all chemically bond.
However, if you use Marine Epoxy as a filler and another branded Epoxy Resin for the fiber mat, it is likely that the two different chemicals do not bond properly and thus do not provide a reliable connection.
To make a reliable and bonding filler, you can just mix the amount of epoxy resin you need with the necessary amount of hardener and then add some saw dust until you have a nice stiff and sticky paste which you can use as a filler. It will bond perfectly with the resin and fiber mat.